Thyroid hormones are divided into two different classes: iodothyronine (thyroxine, triiodothyronine) and calcitonin. Of these two classes of thyroid hormones, thyroxin and triiodothyronine regulate the body's basic metabolism (the level of energy required to sustain the body's vital activity in a state of complete rest), while calcitonin is involved in the regulation of calcium metabolism and bone tissue development.
Thyroid hormones are produced in spherical formations called follicles. Cells of follicles (the so-called A-cells of the thyroid gland) produce thyroxine (T4) and triiodothyronine (T3), which are the main hormones of the thyroid gland. These hormones of the thyroid gland are chemically very similar - they differ only in the number of iodine atoms in the molecule. The thyroxine molecule contains 4 iodine atoms, and the triiodothyronine molecule has 3 atoms (hence the contractions - T4 and T3). Hormones of the thyroid gland are in the blood both in free form and in connection with special protein-carriers. Activity manifests only free forms of thyroid hormones (they are abbreviated as FT4 and FT3, from free T4 - free fraction of the hormone T4).
To synthesize the two main hormones of the thyroid gland, iodine and amino acid tyrosine are needed. In the synthesis, a specific protein, thyroglobulin, is formed, which accumulates in the thyroid follicle cavity and serves as a kind of "reserve" for rapid synthesis of hormones. After isolation into the blood, the thyroid hormones bind to the carrier proteins - thyroxine-binding globulin and albumin. In the free form in the blood there is not more than 0.5% of thyroxine and triiodothyronine.
In the synthesis of thyroid hormones, only pure ("elementary") iodine can participate. Iodine, which enters the thyroid gland, usually has the form of iodide, which then undergoes oxidation and passes into elemental iodine. Subsequently, iodine is included in the amino acid molecule of tyrosine. The addition of one iodine atom to the tyrosine molecule leads to the formation of monoiodotyrosine, two atoms - diiodotyrosine. These compounds do not yet possess the properties inherent in the hormones of the thyroid gland. When two molecules of diiodotyrosine merge, tetraiodothyrosine (thyroxine, T4), a thyroid hormone containing four iodine atoms, is formed. If the molecules of monoiodotyrosine and diiodotyrosine merge, triiodothyrosine (T3) is formed.
In the follicles of the thyroid gland, there is accumulation of a specific protein - thyroglobulin. Thyroglobulin is a kind of stock of thyroid hormones. Thyroglobulin is a glycoprotein with a molecular weight of about 600,000 daltons, the dimensions of which are so great that its entry into the blood from the thyroid gland is almost impossible. Only with diseases of the thyroid gland, accompanied by the destruction of its cells (for example, with the development of thyroiditis - inflammation of the thyroid gland), thyroglobulin gets into the blood.
Calcitonin is produced by parafollicular cells (C-cells) of the thyroid gland, which belong to the diffuse endocrine system. Calcitonin is involved in the regulation of phosphorus-calcium metabolism, enhances the activity of osteoblasts-cells that create new bone tissue. Calcitonin, unlike other thyroid hormones, has a polypeptide structure. It consists of 32 amino acids.
Regulation of the synthesis and secretion of iodine-containing thyroid hormones into the blood is carried out by the pituitary gland, which synthesizes thyroid-stimulating hormone (thyrotropin, TSH). TTG enhances the synthesis of hormones T4 and T3 and their release into the blood. The second important effect of TSH is the growth of the thyroid gland. The intensity of excretion into the blood of TTG is determined by the function of the hypothalamus synthesizing thyrotropin-releasing hormone (TRH). Thus, the production of thyroid hormones occurs in accordance with the needs of the body and is regulated by a complex multi-level mechanism.
The production of thyroid hormones depends on the time of day (the so-called circadian rhythm). Thyrotropin-releasing hormone is produced by the hypothalamus in the morning in the highest concentrations. The concentration of TSH - thyroid-stimulating hormone of the pituitary gland is maximal in the evening and at night. The level of thyroid hormones is maximal in the morning, and in the evening is at the minimum values.
There are also seasonal fluctuations in the level of TSH and thyroid hormones. The concentration of triiodothyronine rises in winter. At the same time, the concentration of thyroid-stimulating hormone increases. The level of T4 - thyroxin significantly during the year does not change. After isolation into the blood, the thyroid hormones are delivered to those tissues of the body where their action is necessary (so-called target cells). In target cells from thyroid hormone T4, one iodine atom (this process is called deiodination) "detaches", resulting in the formation of a much more active hormone, triiodothyronine, which has the main effect. Receptors to thyroid hormones are present in almost all cells of the human body.
Regulation of the synthesis with the involvement of the pituitary and hypothalamus ensures the supply of optimal amounts of thyroid hormones to the blood. With a decrease in the production of hormones by the thyroid gland, the pituitary gland secretes an increased amount of TSH into the blood, as a result of which the function of the thyroid gland increases and the amount of hormones released increases. If the thyroid gland secrete too many hormones, the pituitary gland begins to produce less TSH, resulting in a decrease in the activity of the thyroid gland.
The function of thyroid hormones is to increase protein synthesis, enhance respiratory cell activity, stimulate bone growth and develop the brain. The hormones of the thyroid gland are especially important in the first 3 months of pregnancy, when the active formation of the fetal cortex takes place under their influence. That is why many pregnant women are recommended to determine the level of hormones of the thyroid gland, as well as TTG in order to determine the sufficiency of their synthesis. Long and pronounced decrease in the level of thyroid hormones during pregnancy significantly reduces IQ of a newborn, and also increases the likelihood of complications of pregnancy and premature termination of it. Decreased thyroid function in an adult is called hypothyroidism. The main reason for the decline in the production of thyroid hormones is the defeat of the thyroid cells by the patient's own immune system. As a result of the development of such an autoimmune condition, the number of actively functioning cells is significantly reduced, as a result of which the thyroid gland loses the ability to synthesize hormones in the required amount. Symptoms of a lack of thyroid hormones are the development of edema, dry skin, active hair loss, lower body temperature, slowing mental activity, the development of depression. With a decrease in the level of thyroid hormones, blood supply and oxygen consumption by the brain decrease, the growth and formation of nervous tissue slows down, and the skeleton ossifies. Treatment of this condition is most often performed by the intake of synthesized thyroid hormones, which can safely and effectively eliminate the symptoms of hypothyroidism.
Strengthening of the thyroid gland function occurs when the thyroid gland develops autonomously functioning nodes that produce hormones in excess. In this case, the catabolic action of thyroid hormones begins to prevail - the metabolism is intensified, the stocks of energetically rich substances (glycogen, fat) are actively burned, which leads to an excessive release of heat and an increase in body temperature, sweating, and a rise in the pulse (tachycardia). This condition is called thyrotoxicosis (thyroid hormone poisoning). In thyrotoxicosis, usually the patient's body weight is reduced, but taking thyroid hormone preparations for weight loss should not be - the number of side effects of such "treatment" will be significantly greater than the positive effect. The determination of the level of thyroid hormones is currently carried out by many laboratories, but the techniques used by the laboratories can differ significantly among themselves. The optimal way of counting the number of thyroid hormones in the blood is immunochemiluminescent (the so-called third generation method). The probability of a laboratory error with the use of the immunochemiluminescent method is minimal, and the speed of the analysis, on the contrary, is maximal. At the same time, the cost of the test remains low.