Thyroid Hormone Biochemistry and Physiology

Question 1

Answer 1

• The structures of thyroxine (T4) and triiodothyronine (T3) differ by a single iodine moiety.
• These are the biologically active thyroid hormones, with T3 being the most potent form since it binds 10 times more tightly to its receptor than does T4.
• Reverse T3 (rT3) an inactive metabolite, is found in high concentrations in the fetus and amniotic fluid. In adults, its formation provides a way of lowering the pool of active thyroid hormone to reduce their effects on boosting metabolism in a state of food deprivation.

Question 2

MIT + DIT = […]

DIT + DIT = […]

Answer 2

MIT + DIT = triiodothyronine (T3)

DIT + DIT = thyroxine (T4)

Question 3

Thyroid Hormone

Answer 3

•Thyroid hormone is an important regulator of overall metabolism, and its effects are regulated within a long time frame; a lag time of hours to days for a full effect.

-In fact, it is the most “sluggish” of the endocrine systems.

• Essentially all tissues are to some degree targets for thyroid hormone.
• However, the primary target tissues for thyroid hormone include the liver, kidney, skeletal muscle and heart.

Question 4

Thyroid Hormone Synthesis Step 1: Iodide source and uptake by follicular cells

Answer 4

• Iodine, a trace element, is the key to the thyroid hormones. As there is relatively little iodine in the body or the usual diet, the thyroid gland evolved to both concentrate iodine and make thyroid hormone. Notably, 90% of the body iodine is present in the thyroid, primarily in the organic form as in T4.
• Iodate in the diet is converted to iodide (I- ) in the stomach, absorbed by the intestine and then enters the bloodstream. Iodide in the extracellular fluid is generally very low because of rapid uptake by thyroid cells and by clearance by the kidney.

1) The follicular cell interfaces with the colloid space and the blood.
- Iodide enters the thyroid by a Na+ -cotransporter (symport) driven by the Na+ /K+ -ATPase and induced by TSH
- “iodide trapping”

• Excess iodide in the blood transiently inhibits iodide trapping in the follicular cell (WolffChaikoff effect), iodination and secretion of thyroid hormone. This effect of excess iodide likely protects against short term fluctuations in thyroid function due to dietary iodine
• Dietary deficiency of iodine or defect of the iodide pump lowers thyroid hormone synthesis.
• Certain foods contain natural substances (goitrogens) that interfere with iodine uptake leading to decreased function of the thyroid gland.
• This diminished function is a particular problem in hypothyroid patients.
• Common foods that contain goitrogens include soybeans, peanuts, strawberries, spinach and foods in the cabbage or mustard families.

•Though one might speculate that use of goitrogens might benefit someone who is hyperthyroid, this is in fact ineffective. Patients with hyperthyroidism already have a goiter. Restricting iodine uptake could increase the size of the goiter as the gland attempts to access more blood to increase iodine uptake

Question 5

Thyroid Hormone Synthesis Step 2: Oxidation of iodide

Answer 5

2) Iodide, after entering the follicular cell or following recovery from MIT/DIT, undergoes oxidation.

• Hydrogen peroxide, the oxidant, forms in mitochondria of the follicular cells by an NADPH oxidase reaction that is stimulated by TSH.
• A membrane-bound thyroidal peroxidase uses this H2O2 to convert iodide into oxidized iodide (I+ ) at the luminal surface of the cell.
• The peroxidase is synthesized in the rough endoplasmic reticulum of the follicular cell as a glycoprotein, and transferred to the apical membrane surface via the Golgi where it is incorporated into exocytotic vesicles.
• Synthesis of thyroidal peroxidase also is increased by TSH.

Question 6

Thyroid Hormone Synthesis Step 3: Iodination of Tyrosine

Answer 6

• Tgb, the molecule that is iodinated, is synthesized in the rough endoplasmic reticulum of the follicular cell, and then glycosylated in the Golgi.
• From there it is packaged in exocytotic vesicles that fuse with the apical basement membrane for export to the colloidal space. In the colloidal space, Tgb is iodinated.
• The peroxidase catalyzes iodination of tyr residues on Tgb extracellularly at the brush border (apical surface) that separates thyroid follicular cells from the colloid.
• Iodination of tyr residues in Tgb produces MIT and DIT.

Question 7

Thyroid Hormone Synthesis Step 4: Coupling

Answer 7

• Coupling of two DIT molecules is carried out by the peroxidase enzyme complex, and results in the formation of T4 linked to Tgb.
• A small amount of T3 is similarly formed by linking one MIT and one DIT.
• Although each subunit of Tgb contains ~140 tyr residues, only a small number of these are oriented in a way that allows coupling.
• At least 80% of the thyroid hormone formed is in the T4 form.

-Most T3 (~80%) is produced in target tissues by deiodination of T4.

•These hormone precursors, together with non-coupled DITs and MITs, are stored in the thyroglobulin of the follicular colloid.

Question 8

Thyroid Hormne Synthesis Step 5: Hormone release and iodide recycling

Answer 8

•When thyroid hormone is required, TSH is released from the anterior pituitary in response to TRH (thyrotropin releasing hormone).

-TSH, besides stimulating early steps in the formation of thyroid hormones, increases the number of microvilli on the colloid membrane to facilitate entrapment of thyroglobulins from the colloidal space.

• The thyroglobulin re-enters the follicular cell by pinocytosis.
• The phagosomes, so formed, fuse with lysosomes to form secondary lysosomes.
• In these newly formed structures, proteases degrade the thyroglobulin to liberate the thyroid hormones along with DIT and MIT.
• The MIT and DIT are deiodinated by a thyroid-specific, NADPH-dependent deiodinase.

-In this way iodide is recycled so that only about 30% of the iodide required for each cycle needs to be derived via the thyroidal transporter.

• T4 and small quantities of T3 (<20%) are released by facilitated diffusion to the blood.
• Most of the body’s T3 is not secreted by the thyroid but instead is produced by deiodination of T4 in target cells.
• The colloid contains a reserve of T4 that can last several months.

Question 9

Thyrotropin Releasing Hormone (TRH)

Answer 9

• Regulation of thyroid hormone production involves the TRH-TSH-T4 cascade.
• TRH is released from the paraventricular nuclei of the hypothalamus.
• TRH activates exocytosis of TSH from the thyrotroph via the IP3/Ca2+ and DAG second messengers.
• The thyrotrope is the least common of the cell types in the anterior pituitary though with failure of the thyroid gland there is significant hypertrophy of the thyrotrophs leading to an overall increase in size of the anterior pituitary gland.
• TSH is released in two phases with calcium signaling.

-An initial burst of TSH secretion is followed by a sustained release in response to activated protein kinase C.

•After TRH promotes secretion of TSH, it activates gene activity to replace TSH that had been secreted.

Question 10

Thyroid Stimulating Hormone (TSH)

Answer 10

• TSH (thyrotropin), a tropic hormone produced by the anterior pituitary, promotes all phases of thyroid hormone synthesis and is required for normal maintenance of thyroid tissue.
• TSH stimulates thyroid gland growth and increases vascularity, iodide metabolism, T4 synthesis and release, and promotes Tgb resorption at the cell-colloid interface.
• The TSH receptor couples to Gs so that its effects are mediated via cAMP. Protein kinase A phosphorylates proteins that facilitate thyroid hormone synthesis, pinocytosis and cell growth.
• The thyroid follicular cells contain a high concentration of TSH receptors.

Question 11

Rate of Secretion of TSH

Answer 11

•The rate of secretion of TSH is under a hypothalamic-pituitary-thyroid regulatory axis involving negative feedback.

-Direct stimulation of TSH secretion is controlled by the TRH.

•Inhibition of TSH release is under long loop negative feedback control by the circulating concentration of thyroid hormone, mainly T4.

• When T4 is overproduced by the follicular cell, it feeds back to the hypothalamus and thyrotrophic cells where it is first converted to T3, which then decreases release of and sensitivity to TRH, respectively.
• Consequently TSH release is diminished so that production and release of thyroid hormones by the follicular cells is abated.
• As concentrations of unbound (free) T4 rise in the blood, the rate of secretion of TSH is reduced until T4 content declines.
• In contrast, if the concentration of free T4 decreases, the rate of secretion of TSH increases.
• Additionally, there is a circadian rhythm of TRH and TSH release, with a decrease following the onset of sleep.

-This makes sense because the effects of thyroid hormone on fuel utilization would be unwanted with the lack of fuel intake overnight and also would compete with the effects of growth hormone that is secreted during sleep.

•Overall, the thyroid axis responds rapidly at the hypothalamic-pituitary unit, but beyond this level, the system is governed by processes that have extremely long time constants.

• The long half-life of thyroid hormone in circulation damps the diurnal rhythm that is obvious in TSH concentration.
• As a result, this rhythm is not reflected in circulating thyroid hormone concentrations. This contributes to the “sluggishness” of the system.

•In addition to the feedback regulation of thyroid hormone release, a variety of physiological factors provide control.

Question 12

Answer 12

Question 13

Thyroid Hormone in the Circulation

Answer 13

• Once released into the blood, thyroid hormones bind to several serum binding proteins.
• Consequently, only 0.4% of T3 and 0.04% of T4 are free in the circulation. Only this small unbound fraction of hormone can access the receptors in target cells, and to exert biological activity

Question 14

The 3 Circulating Binding Proteins

Answer 14

• thyroxine binding globulin (TBG) (70%)
• albumin (20%)
• thyroxine-binding prealbumin (TBPA; transthyretin) (10%)

Question 15

TBG

Answer 15

• TBG, a liver-derived glycoprotein, binds thyroid hormone with high affinity at a single binding site.
• The low circulating free concentration of thyroid hormones causes TBG to be only 25% saturated.
• Consequently, a change in concentration of TBG inversely affects the free thyroid concentration whereby decreased TBG increases T4 and T3 and vice versa.

-This relationship differs for steroid hormones and vitamin D because their higher circulating concentration keeps their specific binding globulins saturated.

Question 16

Albumin

Answer 16

•Albumin has a low affinity but a high capacity for T4 being able to carry multiple hormone molecules per albumin molecule.

Question 17

TBPA

Answer 17

•TBPA has a high affinity but a low capacity.. Each of these proteins binds T4, and T3, although the latter with lower affinity.

Question 18

Influence of binding proteins on the dynamic equilibrium of free vs. bound pools

Answer 18

• Thyroid hormones interact with binding proteins in a non-covalent association so that the free and bound hormone pools are in a state of dynamic equilibrium.
• The influence these binding proteins exert on this equilibrium is reflected in their relative half-lives (t1/2) in circulation; T4, 1 week; T3, 1.3 days; rT3, 0.7 days.
• The balance between these two pools may be altered by a variety of conditions.

-For example, high concentrations of estrogens during pregnancy increase production of TBG by the liver. This estrogen effect increases the number of binding sites available in the blood to attract hormone from and decrease the size of the free pool. This decrease in the free pool size is only transient because the system compensates by reducing the rate of hormone removal, and if the imbalance is severe enough, by activating thyroid hormone production and secretion via feedback on the pituitary. The end result is that a new steady state is established in which the free pool is brought back to normal but the bound pool is greater than normal. Though the total hormone pool (bound plus free) is elevated, the free pool (biologically active pool that regulates the feedback) is normal. Hence the mother remains euthyroid (normal thyroid function) without exhibiting symptoms of hyperthyroidism.

Question 19

Peripheral Formation of T3

Answer 19

• Cells take up T4 by passive diffusion or by a specific carrier.
• The deiodination of T4 to T3, in peripheral target tissues is catalyzed by 5’-deiodinase.
• Alternatively T4 can be deiodinated by 5- deiodinase to form reverse T3 (rT3).

-The rT3 is an inactive metabolite that is then successively deiodinated by mono-deiodinases.

• About 40% of circulating T4 is metabolized to T3, and 40% may be converted to rT3 depending on the physiologic circumstance (e.g., well fed versus food deprived states).
• About 70% of T3 and all rT3 in circulation are produced from T4 by peripheral conversion.
• The ratio of circulating T4 to T3 does not reflect the ratio of these two substances released from the thyroid gland. Therefore, the circulating T4 concentration provides a ‘sink’ of prohormone to serve as a ready supply for peripheral conversion to T3 (the most active form).
• When considering overall thyroid hormone biosynthesis and action, attention must be focused beyond the thyroid gland itself. These factors, as well as its long t1/2 in circulation, provide the “sluggishness” appropriate for a hormone that regulates the basal metabolic rate.

Question 20

Physiological Effects of Thyroid Hormone

Answer 20

• Primary determinant of overall metabolic rate – ties to satiety signals
• Effects mediated on:
• metabolism of fuels
• calorigenesis
• growth and development
• CV system
• neuromuscular system

•T4 and T3 treatments = identical effects

-T4 predominant blood form converted peripherally to T3

Question 21

Metabolic Effects of Thyroid Hormone

Answer 21

Question 22

Calorgenic Effects of Thyroid Hormone

Answer 22

Question 23

Growth and Development Effects of Thyroid Hormone

Answer 23

Question 24

Cardiovascular Effect of Thyroid Hormone

Answer 24

Question 25

Food Intake Effect on Thyroid Hormone

Answer 25

Question 26

Mechanism of Action of Thyroid Hormone

Answer 26

• In moderate concentrations thyroid hormone has an anabolic effect, causing an increase in RNA and protein synthesis.
• These effects on synthesis of RNA and protein precede increased basal metabolic rate.
• When administered to patients at higher doses, thyroxine is catabolic, and appears to induce most of the oxidative enzyme systems that have been investigated.
• Thyroid hormone normally has a long latent period, requiring many hours or days to notice an effect.

-For this reason modifying dosages clinically of thyroid hormone is done in small infrequent increments to allow time for the body to achieve a new homeostasis at the new dose.

• T3 acts in target cell nuclei by binding to a chromosomal T3 receptor protein in which the carboxyl end contains the ligand binding domain.
• The receptor protein heterodimerizes with the retinoid X receptor (RXR), also in the nucleus.
• This T3-liganded heterodimeric complex binds to enhancer response elements in DNA via the central domain of the thyroid hormone receptor.
• This binding to DNA stimulates transcription of genes coding for oxidative enzymes (e.g., glycerol 3-P dehydrogenase of the alpha-glycerol phosphate shuttle for NADH to enter the mitochondrion, etc.), growth hormone, beta1-adrenergic receptor, beta3-adrenergic receptor, Na+ ,K+ -ATPase and mitochondrial uncoupling protein (UCP) for heat generation, as well as other functional proteins.
• Its ability to promote synthesis and release of growth hormone may account for some anabolic effects linked to increased amino acid transport and protein synthesis in muscle.