Silicon Nanowire Photovoltaics

In the 14 February 2010 issues of Nature Materials, Kelzenburg et al. report on their progress in developing a photovoltaic cell composed of an array of Silicon nanowires: Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications.  Also see the supplementary information on the fabrication process.

Theoretically an array of nanowires has some advantages over flat or patterned silcon when it comes to light trapping and absorption. The disadvantages come from difficultly in getting the electron-hole pairs out of the wires, both in having sufficient conductivity of the wires and a good electrical contact to the outside world. On the negative side using patterning requires some sort of controlled deposition onto the thin-film substrate when ideally you would like to use some sort of self-assembled system that doesn't require a carefully patterned mask and has a huge through-put for manufacturing.

Figure 1 from (Kelzenburg et al. 2010): (a) SEM image of regular nanowire array embedded in PDMS and (b) schematic of system.

The arrays in question are rods 8-12 μm long that cover 5 % of the areal density of the surface. Let's compare that to a wafer-type Silicon photovoltaic cell, which might have 250 μm of high-quality Silicon in it, the Silicon nanowire PV cell is using about 0.24 % as much material. Since photovoltaic-grade Silicon is quite expensive, this is potentially a cost advantage. From a practical perspective, this concept is robust because the silicon nanowires are embedded in a polymer that protects them from damage.

The authors also added aluminium oxide (Al₂O₃) nanoparticles to the sides of their nanowires in an effort to increase scattering within the cell and hence light trapping. This had a very significant effect and they achieved a maximum of 84.6 % of light absorption compared to 87.2 % for a commercial cell. Remember this is with a tiny fraction of the amount of Silicon used in a commercial cell.

Figure 4 (from Kelzenburg et al., 2010):  Compare the solid red to solid blue lines in (a).  The nanowire arrangement is slightly inferior in the visible spectrum but is markedly superior in the near-infrared.  In (b) area under the curves indicates total light absorption and hence electron generation.

Designing the film so that the nanowires were oriented in a regular, periodic pattern gives a high packing fraction, thereby enhancing light absorption, but it also results in certain orientations being 'dead zones' where light is not well absorbed. In particular, when the rods are facing the sun dead-on, they do not absorb well. However, the regular array of wires would be well suited to situations where the photovoltaic panel was always oriented away from the sun, such as vertical mounts on a wall or horizontal mounts on a flat roof.

Lastly, it was reported at the end of the supplementary information (where better place to hide such details?) that they saw some evidence of sub-bandgap absorption.  That is, light longer than 1120 nm was being absorbed.  This isn't supposed to happen and it tends to reflect parasitic absorption that doesn't contribute to moving electrons (and in fact reduces the output current).  Thus there is some concern that the increase in infrared absorption — the main claim to fame here — was due to parasitic effects rather than something that would actually enhance the electric current being produced. They did not, however, find that all of their cells had sub-bandgap absorption so it may be largely a quality control problem.

Chronic forms of Euthyroid Sick Syndrome and Non-Thyroidal Illness

Introduction

Metabolism is the consumption of energy by a living organism. Basal metabolism is basically the sum of all energy the body uses to maintain its warm internal environment (37 °C or 98.6 °F nominally) in the face of the external environment. In essence, basal metabolism is the energy required to maintain homeostasis, and nothing else. Activity (walking, thinking) is considered to consume additional energy above basal metabolism.

Basal metabolism is measured via the proxy of body temperature and is said to represent the energy used to maintain a constant internal environment, known as homeostasis. Typically basal temperature itself cannot be measured so one measures 'resting' temperature first thing in the morning. For example, part of homeostasis in animals is movement to a more optimal environment, so where do you draw the line? The difference between resting and basal temperatures is a somewhat academic dispute that mostly has to do with the fact that body temperature follows a diurnal pattern, being lower at night while sleeping and higher in the day while active. This difference is a combination of higher cortisol activity and muscle motion during the day. Typically the night-time low cortisol state is regarded as the most reasonable approximation to basal temperature. Furthermore where you measure temperature matters. Armpit temperatures will be lower than sub-lingual (under the tongue) temperatures, which in turn will be lower than rectal or vaginal temperatures. Personally, I have a waking sub-lingual temperature of 36.5-.6 °C measured by a scientific mercury thermometer, which would solidly put me in the normal category at 37 °C internal temperature.

When we are talking about whether someone has a 'fast' or 'slow' metabolic rate we are usually referring to the concentration of active thyroid hormone in the major energy-consuming tissues, and not necessarily body temperature. Although the two are correlated, they are not the same thing. This is not to say that temperature doesn't matter, however, as most of the enzymes in your body have optimal activity close to 37 °C, which is why we evolved to maintain that particular internal temperature.

The thyroid hormone triiodothyronine (T₃) is a DNA transcription effector. That means that every time a cell in your body produces a messenger RNA that goes on to assemble a protein, there must be a T₃ molecule bound to the DNA (typically alongside several other gene regulators). T₃ is an enzyme, so it isn't destroyed by the process of transcription, but the concentration of T₃ is important in determining the rate of protein synthesis. Protein synthesis is a high energy consumption activity (around 9 kcal/g), so someone with poor basal hormone levels is going to have to do something else with that unused energy (i.e. fat storage) and there will also be less waste heat generated, so overall body temperature will decrease. Regulation of T₃ levels is a complicated equilibrium process and I will try to explain it concisely and clear as I'm able.

Regulation of basal metabolism can be broken down into four basic components:

1. the hypothalamus/pituitary gland, which controls the negative feedback of the system and regulates the thyroid through thyrotropin (TSH);
2. the thyroid gland itself, which produces thyroxine (T₄);
3. the deiondinase system (D1, D2, and D3), which converts T₄ to the metabolically active form triiodothyronine (T₃) in many diverse organs but especially the liver, skeletal muscle, the brain, and the thyroid itself, and D3 inactivates T_{3 to} T₂ in the liver;
4. the transport proteins, which are produced by the liver, which regulate the reservoir of T₄ and T₃ found in the blood (and hence their availability to tissues over seconds/hours/days timescales).

The axis of organs which dominate regulation of basal metabolism is the hypothalamus/pituitary/thyroid/liver.  So whenever you have someone who self-diagnoses "hypothyroidism" any one of the above stages in the regulatory chain could be disrupted, and not necessarily the thyroid per se. An individual with poor basal metabolism but a healthy thyroid gland is usually referred to as having euthyroid sick syndrome, with the eu-prefix meaning true or normal. Another common synonym is non-thyroidal illness(es) often abbreviated NTI (de Groot, 1999). Euthyroid sick syndrome sometimes refers specifically to problems downstream of the thyroid (i.e. the deiodinases) and non-thyroidal illness to problems upstream (hypothalamus/pituitary) but the scientific literature is not consistent.

Kohrle (2000), provides an excellent quotation that underscores the complexity of the situation in how the literature on this subject is far too often confusing and contradictory,

As discussed above, complex superimposed and mutually interacting alterations of thyroid hormone economy are observed under these conditions: stress, activation of the pituitary-adrenal axis, inhibition of thyroid hormone production and secretion, changes in serum binding and distribution, tissue uptake and intracellular metabolism. Therefore it is not at all surprising, that different cellular or animal models and experimental manipulations provoking these syndromes led to divergent results.

Nominally non-thyroidal illness is considered an acute condition that passes with time (or kills the patient, acute NTI is very dangerous) but I strongly suspect that the same mechanisms can result in chronic conditions.  My objective in this article is to explore how the basal metabolic system could potentially be disrupted in a chronic fashion to address the extremely common idiopathic “sub-clinical hypothyroidism” that I see evidence of far too often.  The acute form of non-thyroidal illness often features a simultaneous lowering of TSH, T₄, and T₃ but all these features may not be present in more chronic conditions.

Hypothalamus
The headwaters of the basal metabolism control system starts in the hypothalamus, where a number of feedback mechanisms occur. When the hypothalamus determines that basal metabolism should be raised, it releasing thyrotropin-releasing hormone (TRH). TRH modulates the release of a number of other hormones (including prolactin, oxytocin, and arginine vasopressin) but primarily it stimulates the pituitary gland to produce thyrotropin (aka thyroid-stimulating hormone, TSH).

Leptin is one hormone that strongly influences the TRH neurons in the hypothalamus (Rogers, 2009). Individuals who were previously obese still have a greater than normal number of fat cells, with the result that circulating leptin levels are lower than one would otherwise guess from present body fat levels (Spalding, 2008).  I have previously tried to make the case that formally-obese people may develop some of the symptoms of anorexia, in particular hyperactivity paired with lower thermogenesis, and potentially amenorrhea in women. 

Aside from leptin, the TRH portion of the hypothalamus appears to be most sensitive to T₃.  Many tissues have the ability to convert T₃ to T₄, but the hypothalamus isn't one of them. It is reliant on other tissues to produce T₃. Lechan and Fekete (2004) state that,

The source of nuclear T₃ responsible for feedback regulation of TRH neurons in the PVN (RM: paraventricular nucleus, a component of the hypothalamus) differs from the source of T₃ in other regions of the central nervous system (CNS) such as the cerebral cortex and anterior pituitary, where the majority of T₃ arises from the intracellular monodeiodination of T₄ to T₃ by type II iodothyronine 5'-monodeiodinase (D2) (62). This is because the PVN contains little, if any, D2 activity or D2 mRNA (63, 64).

The key take home point is that free T₃ in the bloodstream regulates TSH production (with at least one major exception), while many other parts of the body (e.g. skeletal muscle) which have an active deiodinase system may be more sensitive to T₄, which exists at much higher concentrations.  I will discuss this further later on.

The Lechan group has followed up there results with an effort to see if inflammation can affect the TRH neurons directly (Sanchez, 2008).  Cortisol may be considered the whole-body response to inflammation, and the Sanchez group did not find that it seriously impacted the TRH neuronal activity.  They did, however, find that local inflammatory paracrine hormones (i.e. cytokines, which I like to refer to as immune system catnip) could turn on D2 genes found in specialized neuron supporting cells known as tanycytes (and they were hardly the first group to notice this association).  Specifically, when they injected rats with bacterial lipopolysaccharide, the D2 activity in the tanycytes was turned on, and since these glial cells are adjacent neighbours to the TRH neurons we would expect the local T₃ concentration in the TRH neurons to be higher.  This in turn is going to down-regulate TSH production by the pituitary gland, as well as the other hormones the TRH neurons innervate (prolactin, oxytocin, and arginine vasopressin). 

Lipopolysaccharide is a big word, so to simplify things just note that it is a component of bacterial cell walls, which are considerably different in composition from our cell membranes.  Lipopolysaccharide is considered an endotoxin, and the two words are essentially synonyms as far as immunology is concerned.  The innate immune system (neutrophils, macrophages, and natural-killer cells) can recognize it as hostile, and attacks. In general, I would expect the whole body to respond to an infection via a fever, so at first glance this feedback mechanism seems to be going the wrong way!  However, we have to consider the evolutionary notion that all human cells might respond the same way, since bacteria don’t use thyroid-hormone as a DNA transcription activator.  A bacterial infection in close proximity to the hypothalamus could result in down-regulation of TSH and hence T₄ levels.  If an infection lasts a long time, then overall T₄ levels may be depleted over time and become depressed compared to normal.  This is especially so because the rest of the body's D2 will also increase T₄ to T₃ conversion as a result of inflammatory signals triggered by the infection, thus depleting the T₄ reservoir in the blood stream while simultaneously the thyroid is signaled to produce less T₄. 

Chronic (aka latent) infections are quite possible, especially from organisms that are good at mimicking the host’s biochemistry and avoid total destruction at the hands of adapted immune system. In particular the Herpes family of viruses is well known for causing latent infections (e.g. see Grubor-Bauk, 2008 for where I am going with this), and I have previously made the case for chronic/latent bacterial infection causing atherosclerosis.  Viruses have not yet been shown to cause the same effect, but they still cause a local inflammatory response.  Sachez notes that, “Other mechanisms, such as an increase in proinflammatory cytokines, may be of primary importance in the D2 response to LPS,” so I feel reasonably safe in generalizing this phenomenon to viruses as well.

Interestingly there doesn’t appear to be a specific known system whereby the adapted immune system can recognize the hypothalamus as non-self tissue and attack, i.e. there is no known form of autoimmune hypo-hypothamalitis (it doesn’t exactly roll off the tongue now does it).  Neurons are not supposed to be affected by the adapted immune system (T-cells and B cells), so the dogma says neurons should not suffer from autoimmune diseases.  This is because neurons lack certain surface proteins, namely from the major histocompatibility complex (MHC).  However, natural killer (NK) cells, which are part of the innate immune system but are really a variant of T cells, express different types of surface proteins, such as the KIR class (Cooley, 2006), and they are known to attack neurons and their accompanying cells (Hickey, 1992 and Darlington, 2008).  If I can leave you all hanging for a couple months, I am pretty sure at this point that chronic fatigue and immune dysfunction syndrome/ myalgic encephalomyelitis/ fibromyalgia is precisely this pathology and is inducted largely by Epstein-Barr or cytomegalovirus infections that become latent in the hypothalamus.

Overall the endotoxin/LPS-induced dysfunction of the hypothalamus hypothesis seems to be strongest fit to hormone status for the actual case of acute non-thyroidal illness (de Groot, 2006) so I feel that this is probably one of the strongest potential pathologies for a chronic form of the same.

The Pituitary Gland
The pituitary is located directly underneath the hypothalamus but outside the blood-brain barrier, which enables it to discharge large quantities of peptide hormones into the bloodstream. Although the cell bodies of the TRH neurons are located in the hypothalamus, their axons (which is the long structure that action potentials are fired down) actually snake from the hypothalamus down into the pituitary, where they terminate.  The separation between the TRH neurons and the thyrotropin (TSH) producing cells of the pituitary gland serves a couple of functions. One, the signal produces by the TRH neurons can be greatly amplified by having each of them stimulate many specialized endocrine cells. Second, the pituitary is outside the blood-brain barrier, which facilitates dumping relatively large peptide hormones into the blood. Incidentally, many of the hypothalamic neurons that penetrate into the posterior pituitary are essentially by-passing the blood-brain barrier.  There are also channels from hypothalamus into the anterior pituitary, which is where all the major pituitary hormones are produced. 

The pituitary itself can suffer from degenerative disease such as lymphocytic hypophysitis, which is an autoimmune condition of the pituitary gland (Rivera, 2006 and Crock et al. (Autoimmune Hypophysitis in “Autoimmune Disease in Endocrinology,” ed: A.P. Weetman, Humana Press, Totowa New Jersey (2008)).   It also goes by other names, such as autoimmune hypopituatarism.  There is also an atrophic form where the pituitary is shrunken and scarred by the autoimmune assault, known as granulomatous hypophysitis.  When the pituitary is damaged typically more than one hormone is affected especially as the disease progresses. Adrenocorticotropin (ACTH) deficiency is the most common (60-60 %), followed by thyrotropin (TSH) deficiency (47 %), gonadotroponin (FSH/Lutein) deficiency (42 %), and growth hormone deficiency (42 %).  Prolactin deficiency also manifests (34 %), possibly in conjunction with TSH deficiency.  Headache is often associated with it, although that is a very non-specific symptom.  Approximately 0.5 % of the population appears to be afflicted and I have to say, the quantity of research on this subject is seriously deficient considering just how many people must be affected.  Autoimmune hypopitutarism is strongly associated with autoimmune thyroiditis, with perhaps 40 % of Hashimoto’s patients also having some degree of hypopituitarism. 

I have used Table 1 before, and I present it again because it’s the best way I have to present the many functions of the pituitary gland without a massive wall of text.  In general, when the pituitary isn’t functioning correctly, a huge variety of symptoms can present themselves. 

Table 1: The hypothalamic/pituitary axis hormones and their actions on the human body.

Hypothalamus/Pituitary hormones	Description
Corticotropin-Releasing hormone (CRH) / Adrenocorticotropic hormone (ACTH)	Stimulates the adrenal glands to produce cortisol, a very important general stress hormone that among other things regulates activity of the immune system.
Growth-hormone Releasing Hormone (GHRH) / Growth-Hormone (GH)	Turns on fat metabolism and turns off protein and carbohydrate metabolism, putting the body in a fasted state. Circumstantially may stimulate production of insulin-like growth factor (IGF1) which is responsible for much protein synthesis in bone, skeletal muscle, and many other tissues.
Thyrotropin-Releasing Hormone (TRH) / Thyrotropin (TSH)	Stimulates the thyroid gland to produce T4, the inactive basal metabolism hormone. T3, the active form, is produced by seleno-deiodinases (D1, D2) found in many tissues but in humans predominately the liver and skeletal muscle. T3 is required for the transcription of all proteins (via messenger RNA) from DNA.
Gonadotropin-Releasing Hormone (GnRH) / Follicle-Stimulating Hormone (FSH)	Stimulates the gonads to mature germ cells (eggs and sperm)
Oxytocin (OT) / none	Neuropeptide that down-regulates activity of the amygdala, the anxiety-centre of the brain. Thought to have an important role in social cognition and mood, possibly responsible for “motherly” stereotypical behaviors. Also responsible for uterine contractions during childbirth and menstrual cramps. Can cause spontaneous miscarriage by this mechanism.
Arginine Vasopressin (AVP) / none	Triggers pair-bonding, jealousy, and other ‘male’ stereotypical behaviors. Also acts on the kidney to regulate water retention
Dopamine & TRH / Prolactin Hormone	Dopamine is the Prolactin inhibiting hormone while Thyrotropin-releasing hormone serves a dual role as the stimulating hormone. Prolactin is nominally responsible for lactation during breast feeding but perhaps more interesting is responsible for sexual satisfaction and orgasm in both men and women.
Gonadotropin-Releasing Hormone (GnRH) / Luteinizing Hormone	Triggers ovulation in females, with associated drop in estrogen and rise in progesterone. Triggers release of testosterone in males.

The headache and mass-effect symptoms (i.e. impaired vision) of hypophysitis may be associated with nausea, fatigue, and anorexia (lack of appetite).  Hypophysitis often manifests after childbirth in the post-partum period, and in this case excessive production of prolactin (and hence breast milk) can occur analogous to Grave’s disease (hyperthyroidism).  Lastly, diabetes insipidous is associated with hypophysitis, although lymphocyctic hypophysitis is usually considered to be a disease of the anterior pituitary gland. Diabetes insipidous occurs in the posterior pituitary, and involves dysfunction of arginine vasopressin, otherwise known as anti-diuretic hormone (ADH). 

Diagnosis of hypopituitarism is challenging.  Due to its location in the middle of the skull and small size, the pituitary is dangerous to biopsy.  MRI can sometimes show an enlarged or shrunken pituitary gland.  Low circulating levels of any of the pituitary hormones can indicate hypophysitis but they can also signal other problems with feedback in the hypothalamus or pathology of the hypothalamus proper.  There is an anti-pituitary antibody test for lymphocytic hypophysitis but it is not very specific, likely due to the presence of five major forms of endocrine-hormone releasing tissue in the pituitary.  One of the first groups to survey anti-pituitary antibodies (Stromberg, 1998) only found it in 28 % of their hypophysitis patients although test practices seems to have improved substantially since then by my reading of the literature. The link to celiac disease (below) seems to associate anti-pituitary antibodies with growth hormone deficiency in particular. 

A hallmark of lymphocytic hypophysitis is atrophy of the gonads, adrenals, and thyroid gland.  Endocrine tissue is much like muscle in that you either use it or lose it.  If the pituitary isn’t stimulating these organs, they’ll shrink under the lesser workload, similar to how men who take synthetic testosterone have shrunken testes.  This brings up an aside, that it’s difficult once a patient gets onto hormone-replacement therapy to get them back off, even if the autoimmune reaction is no-longer ongoing. 

One of the more interesting associations of lymphocytic hypophysitis appears to be with celiac disease.  If you are familiar with gluten sensitivity, you are probably aware that there are some idiopathic forms (i.e. those without known pathologies).  An Italian group found that some 40 % of newly diagnosed celiac patients had anti-pituitary antibodies in their blood serum and it resulted in at minimum growth hormone deficiency (Delvecchio, 2010 and my post on the subject). 

Thyroid Gland
Because one can find decent descriptions of hypothyroidism on the internet, I’ll not spend a great deal of time on this section, especially since I am interested in conditions where the thyroid works properly but basal metabolism is still depressed.  The thyroid is a gland located on the throat that, under stimulation by TSH produces Thyroxine (T₄) from tyrosine amino acid residues.  It also needs iodine, namely four atoms per hormone molecule (which is where the four-subscript comes from).  The fabrication of T₄ is affected by two enzymes, thyroid peroxidase (TPO) and thyroglobulin (Tg).

There are two forms of autoimmune thyroiditis: Hashimoto’s, which is lymphocytic, meaning that the thyroid is so packed with immune system cells that it swells and presents as a goiter; and Ord’s, which is the chronic fibrosis form with an atrophied and scarred thyroid gland. Functionally Ord’s and Hashimoto’s appear to have similar outcomes, but present different symptoms.  TPO antibodies typically indicate the goitergenic form while Tg antibodies alone indicate the atrophic form.  TPO antibodies are considerably more common in Hashimoto’s thyroiditis than thyroglobulin antibodies. That unfortunately means Tg-antibodies often aren’t often tested for.  Patients should be aware of this potential for false negative test results in autoimmune hypothyroidism, especially in conjunction with no goiter.  If T₄ is low and TSH is high, make sure to get the Tg-antibody test if the TPO test comes back negative.

With the requirement of iodine, an element that can be in short supply in the diet, many people advocate supplementing with iodine especially if it isn’t taken in the form of iodized salt.  However, I would be remiss if I didn’t mention that there is a strong body of research out there that shows that removing iodine from the diet can arrest hypothyroidism so I would caution people against taking large doses of iodine without being aware of the potential for an adverse outcome (Kasagi, 2003 and Yoon, 2003).

A number of dietary factors can also cause goiter and subclinical hypothyroidism.  The main offender that I see mentioned in the literature is soybean protein products.  The offending isoflavone found in products containing soy protein is called genistein (and to a lesser extent the isoflavone daidzein).  Doerge and Chang (2000) found that genistein irreversibly bound to TPO and stopped its action permanently (in rats).  They did not, however, find that T₄ levels were actually affected.  Rather the production of TSH increased to induce the thyroid to build more TPO.  Similarly another rat study found that the deiondinase system which converts T₄ to T₃ also increased its activity to compensate (Simmen, 2009, please note that no direct effect of genistein on hepatic gene regulation was demonstrated).  Thus I conclude that reasonable quantities of goitrogens are safe for people with healthy basal metabolisms.  On the other hand, if you are hypothyroid, why add additional stress on the system?   This goitrogenic effect could still cause problems in a low-iodine environment, however, as iodine consumption also increased. 

My main concern in this case is that soy (and other goitrogens) might actually induce autoimmune disease.  If genistein binds irreversibly to TPO, it will change the shape/conformation of the TPO somewhat.  If you are unlucky, a receptor on an immune B cell may then recognize the misshapen TPO as a foreign body and launch an autoimmune attack on the thyroid gland.  This is the ‘superantigen’ hypothesis from autoimmunity theory.  The positive news to take from this hypothesis, if you remove the goitrogen, then the autoimmunity should also go away (given half a year or more for the gland to start to heal assuming it hasn’t been completely obliterated by the immune system).

Aside from soy, some of the other goitrogens that I see in the literature include corn, the African staple crops cassava and millet, cruciferous vegetables (cabbage, broccoli, brussel sprouts, etc.), strawberries, and peanuts.  Roman (2007) provides an exhaustive list of potential anti-thyroid agents (including pesticides and other environmental toxins) if the reader is inclined to investigate further.

Selenium Deiodinase Enzymes
Thyroxine (T₄) as produced by the thyroid is not a very effective in up-regulating DNA transcription.  In order for it to be truly effective, one of the four iodine atoms attached to it must be removed, turning it into triiodothyronine (T₃).  There are actually two forms of T₃, depending on which side of the thyroxine molecule the iodine is removed from, and only one of which is biologically active.  The chemically active form has the iodine removed from the left-hand side, while the right-handed form is inactive, and is usually annotated rT₃ (standing for reverse T₃).  The removal of a second iodine results in T₂, which is totally inactive. 

Removal of iodine atoms is called deiodination, and it is accomplished by a set of globular protein enzymes know as the deiodinase family (Bianco, 2003 provides a detailed if technical review).  The deiodinases feature an active site with a selenium atom, which acts to catalytically cleave off iodine atoms from the thyroxine hormone.  Selenium is an essential element required for deiodinase fabrication. There are three known deiondinase enzymes, type 1 and type 2 (abbreviated D1 and D2) both convert T₄ to T₃ and can also deiodinate rT₃, so they cooperate as the on-switches  Type 3 deiondinase (D3) deactivates T₄ to rT₃ and both types of T₃ to T₂, so it acts as the off-switch.

D1 is found predominately in the liver and thyroid, as well as the kidney and pituitary gland and may be present in smaller quantities in other tissues.  It is responsible for most of the circulating T₃, since the thyroid and liver are the primary exporters of thyroid hormone to the body.  About 10 % of the T₄ produced by the thyroid is deiodinated before being released into the blood.  D1 is a simplier complex compared to its cousin D2, and it is capable of producing either T₃ or the inactive form, rT₃, producing roughly equal quantities of both.  The liver is also awesomely good at extracting thyroxine from the bloodstream, a point I will refer to in the thyroid transportation and storage section.   D1 can produce T₃ much faster than D2 can (i.e. it has a high reaction velocity) but it requires a much higher concentration of T₄ which is probably why it is found predominately in the thyroid and liver. 

D2 is predominately found in skeletal muscle, the heart, the brain and other nervous tissue as well as brown adipose tissue.  D2 is apparently found in human thyroid tissue but not rat thyroids.  It seems to have no proclivity to produce the inactive form rT₃, but like D1 it can inactivate rT₃ to T₂.  D2 is capable of functioning at much (~1000x) lower concentrations of T₄ than D1 is (i.e. it has a high affinity for T₄).  Otherwise it is similar in function to D1.

D3 historically was thought to be largely present only during childhood but there is plenty of research now that suggests that is not the case and that it is heavily expressed in the brain and skin (Kestler, 2006).  After all, something has to inactivate T₃ to T₂, a point that some of the review articles I’ve read on the subject seem to have missed.  Other research has pointed to activated immune system cells as potential sources for D3 activity (Boelen, 2008 and Boelen, 2009), which again points a potential finger at chronic infection/inflammation as a possible source of idiopathic hypothyroidism.  One study published in the New England Journal of Medicine found that infantile hemangioma’s could grossly over-express D3 production, resulting in severe hypothyroidism (Huang, 2000).  It’s also well known that placenta tissue expresses D3 heavily to protect the developing infant from the adult’s thyroid hormone levels, which could possibly account for post-partum hypothyroidism, but that is speculation on my part. 

The D2 story is probably the most important from the point of view of a euthyroid sick syndrome. This is because local T₃ concentrations in major tissues are determined by the local gene expression of the deiodinases, and not by blood T₃ levels (Kohrle, 2000).  This is the reason for the heavy emphasis on circulating T₄ in testing for thyroid disorders.  Thus the two principle requirements for maintaining a normal body temperature are:

1. Sufficiently high concentration of circulating T₄ in the blood.
2. Proper gene expression of the deiodinases.

Circulating T₃ is a proxy for D1 activity but not so much for D2 activity.  It does interact with the hypothalamus as a feedback signal, but the majority of thermogenetic (heat producing) activity in the body is going to occur in the big energy-intensive tissues of striated muscle (including heart) (D2), brain (D2), and liver (D1).   If a person has normal T₄ but a low basal temperature, the problem isn’t the thyroid. 

The similar overlapping functions of D1 and D2 begs the question, what is the evolutionary significance of having two enzymes for the same function?  As I’ve mentioned above, D2 seems to express in tissues that are required in survival situations: nervous and muscular tissue primarily.  In comparison, D1 appears in tissues used for digestion (and presumably reproduction).  On such a basis I would hypothesize that high stress will up-regulate D2 expression and down-regulate D1 expression.

Gene expression of the deiondinases is modified by a whole suite of hormones.  Unfortunately, most of the data on this subject comes from rodents who have substantial differences in their deiodinase system compared to humans.  For example, rodents do not produce D2 in their skeletal muscle, but humans do.  I suspect much of the contradictory results in the research surrounding the selenium deiodinase activity are due to interspecies variation, and variation amongst rodent lines.  Nonetheless, I present the data for rodents in Table 2, but please be aware it may be flat-out wrong.

Table 2: General effects of endocrine hormones on D1 and D2 activity in humans (from Bianco and Kohrle)

Hormone	Effect on D1 activity	Effect on D2 activity
Androgens (testosterone)	Increases (tissue specific – liver)	Unknown
Estrogens (estradiol)	Increases (tissue specific	Unknown
Glucocortoroids (cortisol)	Decreases	Trivial increase
Catecholamines (adrenalines)	Unknown	Increases
Growth hormone (GH) & Insulin-like growth factor (IGF-1)	Increases ratio of T3 to T4 and reduces rT3; could also be down-regulating D3	Unknown, likely similar to D1
Thyrotropin (TSH)	Up-regulates thyroid D1 indirectly	Increases
Cytokines (i.e. ‘inflammation’)	Decreases	Increases
Insulin	Increases	Increases
Glucagon	Decreases	Unknown
Cold exposure	Increases	Increases

Reverse T₃ is an interesting molecule that could potentially cause a lot of problems if the balance of T₃ to rT₃ was overly altered.  It acts as a highly competitive inhibitor for the T₃ in DNA transcription, which means it gets into T₃ spot but it doesn’t cause the correct shape changes that allows DNA to be copied into RNA.  It’s likely very difficult to tell rT₃ from T₃ in the lab since they have the same molecular weight and very similar chemistry (e.g. Zhang, 2005), such that your average commercial lab cannot tell the difference. Both D1 and D2 do remove rT₃ by transforming it to T₂ so it is not clear at all whether rT₃ can cause physiological problems.  When considering D1 alone there should be some steady state quantity of rT₃ in tissues that are served by thyroid and liver-derived T₃.  I personally suspect rT₃ is involved in the local regulation of T₃, in that it prevents local tissues that produce T₃ for export to the body from overwhelming their oxygen and nutrient supplies. 

I did find one mention for potential pathology for rT₃ from wikipedia:

rT3, unlike T3, does not stimulate thyroid hormone receptors. However, rT3 nonetheless binds to these receptors, thereby blocking the action of T3. Under stress conditions, the adrenal glands produce excess amounts of cortisol. Cortisol inhibits the conversion of T4 to T3, thus shunting T4 conversion from T3 towards rT3. Consequently, there is a widespread shutdown in T3 binding across the body. This condition is termed Reverse T3 Dominance. It results in reduced body temperature, which slows the action of many enzymes, leading to a clinical syndrome, Multiple Enzyme Dysfunction, which produces the effects seen in hypothyroidism. Effects include fatigue, headache, migraine, PMS, irritability, fluid retention, anxiety and panic.

The only scientific literature I was able to find on this subject was in the Puerto Rico health sciences journal, which seems a strange place for a doctor from Vermont to publish (Friedman, 2006).  Personally, I am inclined to treat Wilson’s thyroid syndrome as a wikipedism for now as I am unclear on how cortisol could directly affect the conversion of T₄ to rT₃ unless it is capable of directly binding to D1 in an allosteric fashion.

One potential link to autoimmunity has been the discovery of anti-D2 peptide antibodies and the fact that they are commonly found in association with anti-pituitary antibodies (Nakahara, 2005).  They found 32 % of their Hashimoto’s patients had anti-pituitary antibodies and 27 % had anti-D2 peptide antibodies, with only a weak correlation between the two (R² = 0.33). The researchers apparently thought that D2 could be the antigen for the anti-pituitary antibody but their results didn’t support their hypothesis very well. This underscores a point that I would like to make: Hashimoto’s thyroiditis may have consequences that extend outside of the thyroid in a large minority of patients, so if T₄ replacement therapy doesn’t provide relief for symptoms, then there may be problems upstream or downstream of the thyroid gland.  Autoimmune diseases have a tendency to cluster. 

From my reading of many of the articles relating to the deiodinases it does seem clear that fasting will decrease thermogenesis (Coppola, 2005) and in particular serum T₃ decreases, but should only do so in a transitory fashion.  I plan to return to this topic at a later date, since it requires a more in-depth discussion than I want to provide in this review. The implication from my point of view is that while circulating T₃ drops during fasting, the brain, heart, and skeletal muscle, which all rely on D2, will be less affected and this seems to lead to hyperactivity, so the net effect on caloric expenditure may be minor.  For most people, fasting remains a good was to restore heath to a damaged liver but if you have low T₃ blood levels avoid fasting as a weight-loss technique.

Transport and Storage of Thyroid Hormones in the Blood
This last section is largely regarding the role of the liver in basal metabolism. Many diseases of the liver are associated with poor thyroid function (Malik, 2002), including cirrhosis.  Malik’s article also makes the point that α-interferon, sometimes used as an antiviral drug, can induce autoimmune diseases including hypothyroidism.  In addition to producing more serum T₃ than any other organ, including the thyroid, the liver is also responsible for fabricating all the proteins that transport thyroid hormone in the blood.

Many hormones in the blood that are not water soluble (especially steroids and thyroid hormones, which are lipids) are often bound to transport proteins, specifically thyroxine-binding globulin (TBG), albumin, and transthyretin (aka prealbumin). Through random chance, some hormones dissociate and some bind such that equilibrium is formed between bound and unbound hormone. Typically only 0.02 % of T₄ and 0.3 % of T₃ in the blood is actually unbound.  Typically 75-80 % is bound to TBG, 15-20 % to transthyretin, and 5-10 % to albumin.

The free hormone hypothesis states that only the unbound (or free) concentrations of thyroid hormones are important to determine concentration inside the cells of body tissues.  However, the three transporting hormones have differing properties and the free hormone hypothesis misses some key points.  First, the binding half-lives for albumin and transthyretin are considerably shorter than TBG, so when in the capillary bed, as the free hormone is taken up by tissues that which is bound to albumin and transthyretin will tend to dissociate to restore the equilibrium (Fig. 1).  This acts to keep the concentration of free thyroid hormone constant as the blood passes through the capillary bed, so that tissues more on the venous side of the circulatory system are not thyroid-hormone deprived.  In this case, the amount of bound thyroid hormone also matters.  This is known as the free hormone transport hypothesis and is almost exclusively the brainchild of Mendel, 1992.  Depending on the length of the capillary bed that feeds them, different tissues may be more or less sensitive to either unbound or bound thyroid-hormone concentrations. 

Figure 1: (A) Dissociation rate of T₄ from transthyretin and, (B) from thyroxine-binding globulin (TBG) (from Mendel, 1988). Typically it takes 60 seconds for blood to circulate.

The liver actually uptakes an amazing amount of the total T₄ in the blood, about 10-12 % per pass. Note this is not 10 - 12 % of the free T₄, but 10-12 % of all T₄ including bound T₄ that passes through the portal vein, which in turn is about 20 % of the blood supply. That means literally the liver is up-taking about two-hundred times the total available free T4, every time the blood circulates past. It doesn’t take a genius to realize that this means that free thyroid hormone levels are not the be all and end all.  As you might expect from this result, the liver has the ability to pick-up thyroid hormone in the bound form (i.e. attached to serum proteins) directly from the blood.

Where Mendel was going with his line of thinking appears to have been that an upset in the balance of binding affinities (i.e. too much TBG compared to transthyretin) could reduce the rate at which T₄ diffuses into the tissues and hence be a potential cause of non-thyroidal illness (Mendel, 1991).  Alternatively other serum molecules in sick patients could bind to the thyroid-binding proteins (either at the binding site or somewhere else), radically changing their affinity to thyroid-hormone such that they hold onto it far too tightly or not at all.  Overall, Mendel’s results were negative for a binding inhibitor but some other research has suggested that free-fatty acids (FFA) as a possible culprit (Chopra, 1986).  Recall that free-fatty acids are often a product of a fatty liver overburdened with fructose intake.  Other research found that bilirubin could act to inhibit uptake of T₄ into the liver (Lim, 1993).  Research in this area seems to have petered out over the last decade; if there are effects, they may be indirect and not causative.

Conclusion

Overall, there is a fair amount of evidence to suggest that autoimmune hypothyroditis may have additional complications beyond the thyroid-gland itself, or that chronic conditions may be resulting in poor basal metabolism even with euthyroid status.  This seems particularly evident for potential problems upstream of the thyroid, in the hypothalamic/pituitary axis.  The deiondinase system is also very likely to be a source of problems, particularly with regard to the many factors that regulate it.  The main trouble with diagnosing problems with the deiodinase system is that it is very complicated and much of the data are contradictory.  This area will continue to develop and perhaps over the next five years the picture will be much clearer than it is now.

This review is not even close to exhaustive; that would require a book. For those of you with unresolved basal metabolism problems I hope I have provided you with some food for thought to try and resolve them. On the other hand, if you are a hypochondriac you probably should have stopped reading at the title. Sorry.

40 % of Celiac Patients have Anti-Pituitary Gland Antibodies

In 2006, an Italian group noted that children with celiac disease often suffered from failure to thrive (i.e. they were short and slight) (Iughetti, 2006). They were able to link this to a deficiency in growth hormone, which is produced by the pituitary gland. Now, growth hormone itself doesn't regulate bone growth, but insulin-like growth factor-1 (IGF-1) does and it is regulated by growth hormone. After removing gluten from the diet, most children had a growth spurt and caught up to their peers but seven did not. Those that didn't catch up were then tested for anti-pituitary antibodies, and five of the seven presented them.

Now 5 of 7 is not a sufficient statistical sample, so the group went back to work over the next three years and tested for anti-pituitary antibodies in many of their newly diagnosed celiac patients (all children) (Delvicchio, 2009, also see editorial by Fasano, 2010). 50 of 119 (42 %) tested positive for anti-pituitary antibodies. There is an interesting comment by Delvicchio to the extent shortness can appear independently of the destruction of intestinal vilae,

It is well established that short stature can be the only presenting clinical feature of CD (31) and in unselected cases admitted for short stature, the prevalence of CD reaches 8%, CD being by far more common than GHD or any other organic disorder. The pathogenesis of CD-associated short stature is still unclear and although growth retardation has traditionally been attributed to generalized or selective malnutrition, new insights on its pathogenesis are emerging.

This suggests that something like one in ten children who are short are short because they eat wheat (notwithstanding selection bias). This isn't the first time autoimmune disease of the pituitary gland has been linked to stunted growth either. A child with lymphocytic hypophysitis (including diabetes insipidus) was previously found to have the same bone-age retardation symptoms as the celiac patients (Weimann, 1997).

The pituitary gland is a small bulbous organ that sits at the base of the brain smack dab in the middle of the skull. The pituitary is often called the master endocrine gland, but it is in turn is controlled by the hypothalamus, a portion of the brain that is the nexus of the autonomic nervous system. The hypothalamus, the posterior pituitary, and the anterior pituitary can all be considered to be different tissues of a single organ system often called the hypothalamic/pituitary axis. Because the pituitary controls such a diverse set of endocrine tissues, if something goes wrong with the hypothalamus/pituitary axis the symptoms can be very diverse. Furthermore, the area is small, located in the center of the skull and hence largely impossible to safely biopsy.

Like other endocrine organs, the pituitary can be victim to the helper and killer T cells of the adapted immune system. This condition is typically called lymphocytic hypophysitis, but also autoimmune hypopitutarism, and typically affects the anterior pituitary which is the hormone-producing portion of the pituitary (Rivera, 2006). The neural portion of the pituitary, the posterior portion, can also be affected and this typically causes diabetes insipidus through arginine vasopressin deficiency (not to be confused with the insulin disorder diabetes mellitus). Autoimmune disease of the pituitary gland is thought to be quite uncommon, with a series of 2500 surgical cases finding an incidence of 0.24 % (6 of 2500) (Sautner, 1995).

Although the Italians appear to have examined primarily growth hormone deficiency, lymphocytic hypophysitis usually results in more than one hormonal deficiency. Rivera (2006) notes in a review for lymphocytic hypophysitis that 60-65 % of cases have ACTH deficiency, 47 % have TSH deficiency, 42 % have gonadotropin deficiency, 37 % growth hormone deficiency, and 34 % prolactin deficiency. For those of you who aren't familiar with the hypothalamus/pituitary hormones I present the following summarizing table:

Table 1: The hypothalamic/pituitary axis hormones and their actions on the human body.

Hypothalamus/Pituitary hormones	Description
Corticotropin-Releasing hormone (CRH) / Adrenocorticotropic hormone (ACTH)	Stimulates the adrenal glands to produce cortisol, a very important general stress hormone that among other things regulates activity of the immune system.
Growth-hormone Releasing Hormone (GHRH) / Growth-Hormone (GH)	Turns on fat metabolism and turns off protein and carbohydrate metabolism, putting the body in a fasted state. Circumstantially may stimulate production of insulin-like growth factor (IGF1) which is responsible for much protein synthesis in bone, skeletal muscle, and many other tissues.
Thyrotropin-Releasing Hormone (TRH) / Thyrotropin (TSH)	Stimulates the thyroid gland to produce T4, the inactive basal metabolism hormone. T3, the active form, is produced by seleno-deiodinases (D1, D2) found in many tissues but in humans predominately the liver and skeletal muscle. T3 is required for the transcription of all proteins (via messenger RNA) from DNA.
Gonadotropin-Releasing Hormone (GnRH) / Follicle-Stimulating Hormone (FSH)	Stimulates the gonads to mature germ cells (eggs and sperm)
Oxytocin (OT) / none	Neuropeptide that down-regulates activity of the amygdala, the anxiety-centre of the brain. Thought to have an important role in social cognition and mood, possibly responsible for “motherly” stereotypical behaviors. Also responsible for uterine contractions during childbirth and menstrual cramps. Can cause spontaneous miscarriage by this mechanism.
Arginine Vasopressin (AVP) / none	Triggers pair-bonding, jealousy, and other ‘male’ stereotypical behaviors. Also acts on the kidney to regulate water retention
Dopamine & TRH / Prolactin Hormone	Dopamine is the Prolactin inhibiting hormone while Thyrotropin-releasing hormone serves a dual role as the stimulating hormone. Prolactin is nominally responsible for lactation during breast feeding but perhaps more interesting is responsible for sexual satisfaction and orgasm in both men and women.
Gonadotropin-Releasing Hormone (GnRH) / Luteinizing Hormone	Triggers ovulation in females, with associated drop in estrogen and rise in progesterone. Triggers release of testosterone in males.

The study did test for thyrotropin (TSH) and didn't find any significant association between anti-pituitary antibodies and basal metabolic dysfunction at the hypothalamus/pituitary axis. However, some 10 % of the patient population did have thyroid disease which is far above the general population average (of around 1 %). Hypophysitis tends to develop over time, with more and more functions being damaged, so these results may simply be representative of the young age of the patients (average age was ~ 6 years old).

In general, persons with one autoimmune disorder are at much higher risk to develop other immune disorders. It would take more effort and text to explain why this is than I'm willing to expand here so I will simply claim this is a fact. It seems that Iughetti has been on the case of co-morbid endocrine disorders associated with celiac disease for quite awhile (Iughetti, 2003). If you go to the Wikipedia page for gluten sensitivity you will see an entire section on "idiopathic gluten-sensitivity." A lot of this could possibly be traced back to autoimmune disease of the pituitary gland.

I recall that something like 12 % of the general population present anti-gliadin antibodies (gliadin being the offending peptide found in the wheat gluten protein). Celiac disease is only one manifestation of gluten sensitivity and I am quite certain that we will continue to see new conditions linked to gluten sensitivity over the next couple of decades. The secondary plant compounds found in the wheat plant's seed seem to be quite harmful to humans and in my opinion wheat is not suitable for human consumption. Plants, after all, cannot run away so they must rely on armour and toxins to stop animals from eating them. Anyone suffering from an unknown condition that presents as a disrupted hormonal milleu should strongly consider the shotgun approach of a paleolithic diet as a potential solution.

Degenerative and man-made diseases in the developing world

I came across a relatively recent article by Kuper and Kuper in the Financial Times on the rise of degenerative and man-made diseases in the developing world that I suggest reading. The advent of smoking and low-quality industrial food is making many of the world's poor less healthy even as medicine manages to fight back infectious diseases around the globe.

This paragraph, however, made me laugh:

We now know that Omran failed to foresee a fourth stage of the transition: the decline of chronic diseases. The west – and particularly its richest inhabitants – has now reached this stage. Thanks to the “cardiovascular revolution” – the medical advances in treatment – the past 30 years have seen death rates from heart disease fall by 70 per cent in the US, the UK, Australia, Canada and Japan. That translates as 14 million American and eight million British lives saved between 1970 and 2000.

Part of the problem with modern medicine is that mortality trumps morbidity, every time in funding, in research effort, and in much every aspect of the system. Eventually I think the system will figure things out, but there's a lot of inertia to overcome as well as moneyed interests who benefit from people in poor health. The whole article is really a great example of the common wisdom facilities that need to be overturned so we can get anywhere.

I have been a little snowed under lately with work so no substantive posts. I am going surfing in Tofino for reading week, maybe I will work up a post while I am away, or maybe not...

Foxa2 Transcription Factor Implicates Direct Role for Insulin in Hunger

Insulin has a number of down-stream targets (receptors), one of which has the highly descriptive name foxa2 (aka hepatocyte nuclear factor 3-beta). This receptor is closely tied to the liver and how it reacts to insulin (Wolfrum, 2004). Essentially foxa2 is one of the regulators responsible for the production of enzymes that are involved in beta-oxidation of fatty acids and the production of ketones. Insulin binds to foxa2, making it unable to perform its function in activating the production of these important fatty-acid metabolism enzymes. In this manner, insulin shuts down fatty acid metabolism.

First a little background on gene expression if you didn't implicitly understand what I wrote above. About 1 % or so of your DNA encodes for proteins. To make a protein, the DNA has to be pulled apart and a complementary single-stranded RNA polymer built to match. This is called messenger RNA and it is sent off to an organelle known as a ribosome which actually assembles the protein that the DNA encodes for.

The rest of your DNA is functional with 'dead space' being a common function. DNA, when being unzipped typically folds in on itself, so sections of the DNA well ahead of the protein encoding region often have functions related whether or not that protein encoding region is actively being transcribed or not. Other proteins literally sit on these spaces and their interaction with the enzymes that unzip and transcribe DNA determine whether messenger RNA is produced or not. Foxa2 is one of these gene transcription activators, so it operates at a very basic level of cellular mechanics. However, it can only do this when insulin is bound to foxa2. Presumably the binding of insulin to foxa2 reconfigures the shape of the foxa2 protein; with proteins, function follows structure/shape.

Silva et al. (2009) published in the journal Nature that this same receptor, foxa2, is found in the hypothalamus (of mice) and it directly effects the hunger reflex. What they did was to take normal and genetically obese mice, fast them, and inject some of them with insulin to put them into the 'fed' state. They then sacrificed the mice and dissected their brains, using an antibody-based stain to identify neurons that were positive for foxa2 and orexin and melanin-concentrating hormone (MCH). Orexin and MCH are known to be associated with feeding and, incidentally, sleep behaviour (Willie, 2001). From their results the authors concluded that the production of the neuropeptides orexin and melanin-concentrating hormone (MCH) were promoted by the foxa2 receptor (but only with insulin attached to it). One of the stronger pieces of evidence was that foxa2 was found in the cytoplasm of the mouse neurons when in the fasted state but in the nucleus when in the fed state. Transcription of messenger RNA occurs in the nucleus.

Supplementary Figure 1 (from Silva, 2009): Author's impression on how the foxa2 receptor cycles in and out of the nucleus in response to insulin.

I put mice in parenthesis in the preceding paragraph because there have been significant differences found between how various pieces of molecular machinery are distributed in rodents versus humans. A good example of this is selano-deiondinase type 2 (D2), which converts the inactive form of thyroid hormone, T4, into the active form T3. It's found in the skeletal muscle of humans but not rats ( Heemstra, 2009 and Larsen, 2009). Incidentally, both papers have fascinating implications for fasting in humans as well as sick euthyroid syndrome.

Interestingly the way they found the distribution of the Foxa2 receptor amongst neurons of the hypothalamus was through the use of specific antibodies as a microscopy fluorescence stain. I'm not clear on how these antibodies are fabricated from the supplemental literature, and a search for, "foxa antibod*" on PubMed didn't return any pertinent hits. I'm sure this is a common method as I've seen it before in fluorescence microscopy, but I am curious about the potential for an associated autoimmune disease.

If you read my previous post on leptin and anorexia (01/12/2010), there is additional support in Silva for the notion of hyperactivity in a fasted state:

Interestingly, the Nes-Cre/+;Foxa2T156A^flox/flox allele was associated with dramatic increases in spontaneous locomotor activity relative to control mice (Fig. 3i). The difference between the locomotor activity of Nes-Cre/+;Foxa2T156A^flox/flox mice and that of Foxa2T156A^flox/flox or Nes-Cre/+ mice was similar to the increase in movement of fasted wild-type mice relative to fed wild-type mice (Fig. 3j). The types of physical activity induced in Nes-Cre/+;Foxa2T156A^flox/flox mice included searching as well as intense grooming, rearing and face-washing behaviour.

If you can get past the ridiculous names of the mice variants, the English here is pretty clear.

Now it's very easy to get lost in minutiae such as this and lose clarity in the process. Of course, minutiae does have value for the task of bamboozlement. If we pull back and look at the big picture, the key point here is that insulin has been directly implicated in the hunger reflex for the first time, to my knowledge. Previously I assumed that only leptin and ghrelin can effect hunger. Now, when I read this article I did ask the question, has insulin been implicated to interact with this receptor at a biochemical level, or perhaps it is stimulating some other intermediate hormone which in turn interacts with foxa2? The answer is, yes, insulin is the actor and it directly binds to foxa2 (Wolfrum, 2003).

Leptin and Anorexia

I have, off and on, entered into discussions with other bloggers on the role of leptin in long-term energy storage. Leptin, we know, is strongly related to long-term storage of fat and is probably one of the primary hormones associated with obesity (Kelesidis, 2006). It is thought, along with ghrelin, to be one of the hormones responsible for appetite.

One question I've posed is does leptin have an antagonist hormone? Most hormones have complements that act to oppose their action. For example, insulin versus glucagon/growth hormone. As an aside, please recall that growth hormone is primarily a catabolic hormone that turns on the body's fat metabolism, a state we call fasted. Generally an antagonist allows the endocrine system to respond more rapidly than simply waiting for the pertinent hormone's concentration in the blood to clear. Does leptin need an antagonist? Or does it operate over such a long time-span that it normally wouldn't need one? Is the lack of an (apparent) antagonist perhaps one of the reasons leptin metabolism can go screwy?

As an alternative to looking into why leptin makes people fat, I thought it might be interesting to examine how a lack of leptin makes people skinny, or anorexic. Anorexia just means 'skinny' refers to a lack of appetite in medical parlance, while anorexia nervosa (AN) refers specifically to the eating disorder that we've all heard about in the news. People can have abnormally low-body fat without having an eating disorder. For example, individuals with cortisol insufficiency (such as Addison's disease, an autoimmune condition involving destruction of the adrenal cortex) tend to have very low body fat levels, but not necessarily a lack of lean body mass. The lack of cortisol just mutes the body's stress response to store an emergency reserve of fat.

One of the markers that characterizes anorexia nervosa is low circulating leptin levels.

Now, leptin likes to interface with the hypothalamus, which is the part of your brain that essentially acts as an interface between the digital-fast (neural) and analogue-slow (endocrine) control systems of the human body. Lot's of things like to interface with the hypothalamus though, so please do not take this role of leptin as dogma. Together, the hypothalamus and pituitary are the master endocrine organ system, regulating the serum concentration of most of the hormones in your body. Essentially it integrates many different signals, and based on those signals decides what quantity of eight primary hormones to release (i.e. oxytocin, argigine vassopressin, adrenocorticotropic hormone, growth-hormone, thyrotropin (TSH), prolactin, luteinizing hormone, and follicle-stimulating hormone). The hypothalamus plays a crucial role in regulating immune function, metabolism, sex function, and mood/anxiety amongst many others.

The hypothalamus (and the pituitary by extension) tends to release hormones in pulses. When I say the hypothalamus exists on the border between digital and analogue that is nearly literally true. The hypothalamus samples the blood-stream for various feedback mechanisms (i.e. hormones) and when it adds together enough signals that indicate the system needs more growth hormone, it generates a pulse. This is done by the combination of neural and endocrine tissues. Leptin is one of the signals that contributes to whether or not pulses are released from the hypothalamus. If everyone's leptin receptor cells are identical, which is not likely, then low leptin levels will probably down-regulate some of the hypothalamic-pituitary hormones and up-regulate some others, while high leptin levels will do the opposite.

One very common side-effect of AN is the loss of the menstrual cycle (which has the scientific name amenorrhea) The menstrual cycle is initiated by a luteinizing hormone pulse, which implies that very low leptin levels have effects beyond simply regulating fat levels. This is not a surprising result; we would expect the body to shut down non-essential functions when it is starving. This result is correlated to circulating leptin levels (Blüher, 2007). Blüher has some interesting comments on the matter of leptin release:

Leptin secretion can be stimulated by insulin, glucocorticoids (RM: cortisol), and cytokines (RM: immune system catnip) (i.e. tumor necrosis factor [alpha]), whereas catecholamines (RM: "adrenaline"), free fatty acids, cold exposure and thyroid hormones inhibit leptin release [18,19]. Estrogens induce leptin production whereas androgens (RM: male sex hormones) suppress it, providing an explanation for the sexual dimorphism in serum leptin levels [19]. Although anthropometric and clinical features (gender, fat mass/fat distribution, hormones and cytokines) may influence the secretion pattern of leptin, the crucial factor in regulating serum leptin levels seems to be caloric intake and the amount of energy stored in adipocytes [5].

Another side-effect of AN is increased activity (aka hyperactivity), which is a homeostatic method to increase caloric expenditure. This is called activity-based anorexia (ABA) and is one of the primary animal models of anorexia. A review by Hillebrand et al. (2008) shows that leptin itself appears to be signaling the hypothalamus to encourage the brain to engage in this sort of behaviour, and that leptin-replacement therapy suppressed this activity. It's been hypothesized that hyperactivity would promote foraging behaviour in the paleolithic-era and in wild animals. Leptin also has a role in the homeostatic mechanisms behind thermogenesis via the basal metabolism of the thyroid hormones and brown adipose tissue (Rogers, 2009).

This result begs the question, are obese individuals sedentary because they have high circulating leptin levels? Was Gary Taubes, of Good Calories, Bad Calories fame, right in the lack of a relationship between exercise and obesity, even if he didn't know why? If so, hyper/hypoactivity as it relates to leptin would appear to be a case of positive feedback, where the signal tends to reinforce itself over time. It's only because gathering food requires so little energy investment today (get off couch, walk to pantry, grab chips) that this positive feedback cycle blows up so spectacularly. Historically putting on some fat might discourage activity via leptin, giving the organism a rest period.

Now on another front, anorexia nervosa patients who recover from the condition and regain body weight often regain too much and become overweight. This occurred even when caloric-intake and leptin levels were monitored during the body weight gain period to prevent excessive weight gain (Lob, 2003). So once again we see the dominance of the endocrine system and homeostasis over counting calories.

What might cause this higher than normal set-point of body mass index (BMI)? This question does not seem to have a firm answer quite yet so I'm going to speculate. The hypothalamus is a union of neural and endocrine tissue. Neurons, in particular, are quite plastic in that the amount of stimulus you have to apply to get them to fire changes depending on their exposure history. This is how memory is thought to work, for example. My hypothesis is that the neural component of the hypothalamus habituates to long-term leptin exposure.

There are clearly some threshold levels where leptin indicates an organism is in semi-starvation mode and generates compensatory behaviour (Müller, 2009). I can postulate that there may also be hibernation morphology at the top-end of the leptin spectrum. If the organism stays in semi-starvation mode for long enough, perhaps the sensitivity to leptin in the hypothalamus is reduced by the plasticity of the neural component. In this case, a crash weight-gain diet would not give the hypothalamus's neurons sufficient time to change their sensitivity to leptin, and adapt a new set point.

Maybe this is the reason why fast weight-loss programs typically fail miserably. The leptin set points for semi-starvation modes are at at abnormal levels, and pushing leptin through them induces behaviour that likely results in a rebound. The solution then is to be patient and go slow with weight loss or gain. If my hypothesis is correct, losing weight too fast may actually permanently distort leptin regulation.