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Functional status of the thyroid gland
Last reviewed: 05.07.2025
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Thyroid diseases are the second most common endocrine diseases after diabetes mellitus. They develop as a result ofthyroid function disorders, changes in the biosynthesis of thyroid hormones or their action in tissues.
Inorganic iodine and the amino acid tyrosine are required to form thyroid hormones. Every day, 30-40% of iodine consumed with food is concentrated in the thyroid gland along with iodine formed as a result of peripheral destruction of thyroid hormones. The remaining iodine is excreted in the urine. In the body, it is in the form of inorganic iodine and in a protein-bound form. When necessary, iodine is captured by the thyroid gland and oxidized into molecular iodine, which combines with a specific protein - thyroglobulin. 1-2% of iodine remains in free form. Iodine is concentrated in the thyroid gland both in the colloid of follicles and in epithelial cells. Proteolytic cleavage of thyroglobulin leads to the release of T 4 and T 3, as well as the release of iodinated amino acids - mono- and diiodotyrosine. T4 and T3 in the blood are reversibly bound to a specific protein - thyroxine-binding globulin (TBG). When the content of thyroid hormones increases, the excess binds to other proteins - prealbuminand albumin . A balance is created in the blood between bound and free hormones.T4 and T3 bound to protein represent a kind of hormone depot, from which they are released as needed. Only free hormones in the blood have a biological effect.
The thyroid gland function is under the control of TRH secreted by the hypothalamus.TSH secretion is stimulated by TRH, which, released by hypothalamic cells, binds to membrane receptors of pituitary cells, activating adenylate cyclase and causing proliferation of glandular cells of the adenohypophysis. Under the influence of TSH, thyroglobulin passes into the follicular cells of the thyroid gland, then is hydrolyzed by proteolytic enzymes with the formation of T4 and T3 . Changes in the sensitivity of adenohypophysis thyrotrophs to the stimulating effect of TRH depending on the concentration of free thyroid hormones in the blood are the main mechanism for regulating thyroid function.
Thyroid hormones affect various metabolic processes in the body. They increase the utilization of carbohydrates, potentiating the action of insulin, and increase the absorption of glucose by muscles. In physiological quantities, thyroid hormones stimulate protein synthesis, including the synthesis of specific enzymes; increase lipolysis and oxidation of fatty acids; and potentiate the action of some hormones.
Dysfunction of the thyroid gland leads to the development of symptoms caused by metabolic disorders.
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