What Is Thyroid Hormone Biosynthesis?

Thyroid hormone is a hormone secreted by the thyroid gland, which acts on almost all cells of the human body. English name: thyroxin (e) Alias: dry thyroid, abbreviation: TH thyroid powder molecular formula: C15H11O4I4N relative molecular mass: 776.93. Physical properties: white needle-like crystals. No smell. Tasteless. Deterioration in case of light. Melting point 231-233 ° C (decomposed). Insoluble in common organic solvents such as water and ethanol. Soluble in ethanol containing inorganic acids or bases, but also in alkali hydroxide and carbonate solutions. Add sodium nitrite to its acidic ethanol solution, it will turn yellow when heated, and it will turn pink by adding excess ammonia. Chemical nature: Thyroxine is T4, which is tetraiodothyronine. Preparation: It can be extracted from the thyroid of cattle, sheep, pigs, etc. or artificially synthesized.

Thyroid hormone

Thyroid hormone is a hormone secreted by the thyroid gland, which acts on almost all cells of the human body. English name: thyroxin (e) Alias: dry thyroid, abbreviation: TH

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The biological effects of thyroid hormones are as follows:

Tetrodine

Formation of thyroxine

Thyroid hormone is unique in that its biological activity requires the trace element iodine. In most parts of the world, iodine is a rare component in the soil, so iodine is scarce in food. Biology has formed a complex mechanism in evolution to obtain and retain this key element, and transform it into a form suitable for incorporation of organic ingredients. At the same time, the thyroid gland must synthesize thyroxine, which occurs in thyroglobulin.

Thyroxine is formed through six processes: synthesis, storage, iodination, reabsorption, decomposition and release:

1.

Thyroid stimulating hormone (TSH) secreted by the pituitary gland promotes the entire process of thyroid hormone synthesis and secretion, and the secretion of TSH is regulated by the thyortropin releasing hormone (TRH) secreted by the hypothalamus. Stress, changes in ambient temperature, and certain diseases all affect thyroid function through TRH. On the other hand, T4 and T3 concentrations in the blood have a negative feedback regulating effect on the release of TSH and TRH. ^[4]

Thyroid hormones include: thyroxine and triiodine

T3 is fast and strong, T4 is weak and slow. Thyroid hormone binds to its receptor in the nucleus and induces the target

Physiological effects and metabolism

1. Thyroid hormone production

T4 is the highest content of iodide in Tg

1. Long-term excess can cause

Thyroid hormones have very important effects on the nervous system. The normal development and maturation of nerve cells depends on the action of thyroid hormones. Embryonic stage to

Thyroid hormone is synthesized, stored and released by the thyroid gland. The raw materials for thyroid hormone synthesis are iodine and tyrosine in the body. Under normal diet, the human body consumes 100 to 200 micrograms of iodine per day. Intestinal absorption of iodine is complete. Iodine in the diet is first converted to iodide on the intestinal mucosa and absorbed; skin, mucous membranes and lungs can also absorb iodine, but it is much worse than intestinal absorption. The intestinal absorption of iodine is mainly distributed in extracellular fluid. The iodide concentration in the serum was 0.5 g%. The thyroid has the ability to concentrate iodine. The iodine concentration in the thyroid is 20 to 40 times higher than the iodine concentration in the blood. Patients with hyperthyroidism can increase concentration by 100 to 200 times. The salivary glands are derived from the foregut as the thyroid gland, so the salivary glands also have the function of concentrating iodine. The ratio of normal saliva iodine / serum iodine was 20. Clinically, the ability of the thyroid to concentrate iodine can be easily determined by measuring the content of salivary gland iodide. Except for the salivary glands in the body, other glands also have the ability to concentrate iodine than the thyroid gland. Under normal circumstances, iodine in saliva and gastric juice is reabsorbed by the digestive tract and returned to extracellular fluid.

The thyroid concentration of iodine is concentrated from low concentration to high concentration. It is an active concentration function and requires energy consumption. The thyroid's ability to concentrate iodine is mainly stimulated by the pituitary thyroid stimulating hormone (TSH) and is also inhibited by high concentrations of iodide in the body. The higher the thyroid-stimulating hormone, the stronger the ability of the thyroid to concentrate iodine; the higher the concentration of iodine in the blood, the lower the ability of the thyroid to concentrate iodine.

There is iodine in the thyroid follicular epithelium, which is converted into active iodine by the action of peroxidase, and quickly synthesizes monoiodotyrosine (T1) and diiodotyrosine with tyrosine on thyroglobulin. (T2). Two diiodotyrosines are coupled to thyroxine (T4), one diiodotyrosine and one monoiodotyrosine are coupled to one triiodotyrosine (also known as triiodoadenosine) Acid-T3). Thyroid hormones synthesized on the surface of thyroglobulin are stored in the glial follicles. The main component of the thyroid follicular cavity is thyroglobulin. The thyroid hormone stored in the thyroid follicle cavity can be used by the body for 2 to 3 months. The synthesis of thyroid hormone occurs on the surface of the thyroglobulin. The thyroid is the only endocrine organ in the body that stores hormones outside the cell. Other endocrine organs and glands store hormones in cells.

When the body needs it, the thyroid follicle absorbs colloids in the follicle cavity into the follicles to form colloidal droplets through the effect of cytotoxicity, and combines with lysosomes to form phagolysosomes. The lysosome contains proteolytic enzymes and peptidases. T4 and T3 are hydrolyzed from thyroglobulin and released into the blood. The lysosomal enzyme hydrolyzes monoiodotyrosine and diiodotyrosine when it hydrolyzes T4 and T3 on thyroglobulin, and further releases free inorganic iodine under the action of deiodinase. The latter are mostly re-used by follicular cells to synthesize thyroid hormones. This is important for the thyroid to use the limited iodine in the body economically.

(3) The vast majority of thyroid hormones in the blood bind to proteins in the plasma, mainly to thyroid binding globulin (TBG), and also to plasma proteins (ALB) and thyroid pre-albumin (TBPA). T4 (75%) is mainly combined with TBG, T3 (90%) is mainly combined with thyroid binding globulin and plasma albumin; 99.97% of T4 in the blood circulation is bound, and 99.7% of T3 is bound. Although bound thyroid hormones account for the vast majority in the blood, it is still the free thyroid hormones that really play a physiological role. Thyroid binding globulin is a single-chain glycoprotein produced by the liver and has a half-life of 5 to 6 days. It carries 70% of T4 and T3; thyroid-binding prealbumin is produced by the liver and has a half-life of 1 to 2 days. It has a lower affinity for T4 than thyroid-binding globulin for T4, and it carries 10% to 20%. T4, hardly carry T3. The plasma albumin concentration is high, which can reach 3000 to 4500 ml / dl, but it has the lowest affinity for thyroid hormones, and can only carry 5% to 15% of T4 and 30% to 50% of T3. Some factors that affect thyroid binding protein will affect thyroid hormone levels, but free thyroid hormone levels are normal and thyroid function is normal. When the real thyroid is hyperthyroidism or decreased, the serum total thyroid hormone and free thyroid hormone are like articles. Thyroid binding protein is like a "warehouse" one by one. There are countless "warehouses" in our body. Most of the items produced by the "factory" are stored in the "warehouse" first, and a few are stored in the "store". These items in the "shop" are available for purchase, but most of the items in the "warehouse" are not available. Because of the powerful "warehouse" items as backing, the items in the "shop" are always maintained at a very stable level. Although there are many items in the "warehouse", they are not in circulation and cannot function. And what is really valuable and can play a physiological role are those items placed in the "shop". The size of the "warehouse" can affect the number of our items, but does not affect the items in the "store". A large number of bound thyroid hormones are stored in the blood, which prevents a large amount of thyroid hormones from being lost from the urine when the blood is filtered from the kidneys, and ensures that iodine is more economically used in the human evolution process from iodine-rich oceans to relatively iodine-deficient land. Secondly, a large number of bound thyroid hormones are present in the body, maintaining the stability of the thyroid hormones day and night, and ensuring the continued need for thyroid hormones in the body's metabolism.

T4 binds to protein tightly and clears slowly, with a half-life in plasma of 7 days; T3 binds relatively loosely to protein, clears quickly in plasma, and has a half-life of 1 day. 100% of T4 in plasma comes from thyroid, T320% in plasma comes from direct secretion of thyroid, 80% comes from peripheral transformation of T4 in plasma, and more than 95% of rT3 (reverse T3) comes from peripheral deiodination of T4. So what really represents thyroid function should be serum T4 concentration, not T3 concentration.

Metabolism of thyroid hormone in the body is directly filtered from the kidney, and part of it is excreted after being degraded in the body. Its degradation pathway in the body: excreted after deiodination; excreted after degradation by the side chain; degradation of thyroid hormone mainly occurs in the liver, and is also degraded and metabolized in surrounding muscle tissue to generate inactive iodide. T4 removes 1 iodine from the outer ring to generate T3 after deiodinase action; it can also remove 1 iodine to produce reverse T3 (rT3) at 5 by deiodinase action. The biological activity of T3 is very strong, and the activity of rT3 is very low. When suffering from serious diseases, chronic hunger, malnutrition, and taking certain drugs such as propylthiouracil, steroids and propranolol, 5, deiodinase activity is inhibited. 5. Deiodinase activity increases, showing that serum T3 decreases and rT3 increases, which is a reaction that the body protects itself under severe conditions. When the body returned to normal, 5, deiodinase activity returned to normal, and serum T3 and rT3 also returned to normal. Thyroid hormone metabolites are mainly excreted through the kidney, a small part is excreted into the intestine through the bile, and then re-entered into the blood through the liver-gut circulation for reuse. Urinary iodine content accounts for about 90% of our iodine intake, so we can estimate the daily iodine intake by measuring the urinary iodine content.

In addition to deiodine metabolism, thyroid hormone has other metabolic pathways, such as T4 and T3 can be oxidized and deiodinated to produce tetraiodothyronine and triiodothyronine; T4 is combined with glucuronic acid and T3 is combined with sulfate. In vitro excretion