Thyrotropin, also called thyroid-stimulating hormone, is a control hormone that regulates thyroid activity, hormonal production, and growth. It is secreted and regulated through interaction with other hormones. Overproduction or underproduction has far-reaching effects on thyroid function.
What is thyrotropin?
Infographic on the anatomy and location of the thyroid gland, as well as the symptoms of hyperthyroidism and hypothyroidism. Click image to enlarge. Thyrotropin belongs to the group of hormones, these are biochemical substances that act on specific cells or organs and thus participate in the regulation of circulation and other functions of the organism. In the case of thyrotropin, this regulated organ is the thyroid gland. Chemically, thyrotropin is a glycoprotein, i.e. a macromolecule consisting of a protein with covalently attached carbohydrate groups. This protein consists of two subunits called the alpha and beta subunits. They differ in the number of amino acids they contain. The beta subunit, which contains 112 amino acids, is specific to this hormone, while the alpha subunit, with its 92 amino acids, is found in similar form in other related hormones. These related hormones include human chorionic gonadotropin, follicle-stimulating hormone, and luteinizing hormone.
Production, formation, and manufacture
Thyrotropin is synthesized in the thyrotropic cells of the anterior pituitary. This is an area of the diencephalon that is responsible for the control of the autonomic nervous system, among other functions. The synthesis of thyrotropin is controlled by complex interactions of various other hormones. The most important of these hormones is thyroliberin. This is formed in the hypothalamus, also an important component of the diencephalon, and then travels to the anterior pituitary through a special vascular system. Depending on the concentration of thyroliberin, this stimulates the production and release of thyrotropin. Interaction with thyroid hormones, on the other hand, can lead to suppression of thyroleiberine formation, which also affects thyrotropin synthesis and release.
Function, action, and properties
After release in the hypothalamus, thyrotropin is released into the bloodstream. One of its functions is the controlled release of lipids in adipose tissue. However, the actual target organ is the thyroid gland. Here thyrotropin acts on the cells of the thyroid gland by stimulating increased cell division. Iodine uptake by the thyroid gland also increases. This function of thyrotropin to increase iodine uptake by tissues is exploited in the treatment of thyroid cancer. Special drugs containing an artificially produced form of thyrotropin, called recombinant human thyrotropin, are used in radioiodine therapy. Radioactive iodine is thus absorbed more quickly by the diseased cells. As a further effect of thyrotropin on thyroid function, the production of the thyroid hormones thyroxine and triiodothyronine is increased. These two peripheral hormones have a high iodine content and are also essential for energy metabolism. Thus, they are involved as reactants in important reactions such as glycolysis and gluconeogenesis. Through negative feedback, they also suppress the formation of thyrooliberin if the concentration is high enough. This indirectly inhibits the synthesis of thyrotropin. In contrast, at low concentrations of peripheral thyroid hormones, the formation of thyrooliberin is stimulated. This ensures that the concentration of thyroid hormones is always appropriate to the demand, because production is very energy-consuming. This balance of thyroid hormones can be disturbed by increased or decreased production of thyrotropin.
Diseases, ailments, and disorders
When the production and secretion of thyrotropin is too low, a condition called pituitary hypothyroidism occurs. The thyroid gland can no longer absorb iodine or produce thyroid hormones. As a result, growth is suppressed and the thyroid gland becomes progressively smaller. This can lead to atrophy. In this case, we speak of secondary hypothyroidism.In the case of increased production and secretion of thyrotropin, on the other hand, the iodine uptake and hormone production of the thyroid gland is pathologically increased. The cause is often an adenoma, a benign growth of the tissue responsible for the increased thyrotropin production. The increased production of thyroid hormones is called pituitary hyperthyroidism or secondary hyperthyroidism. These two forms of disease presented are called secondary because they are not caused by changes in the thyroid gland itself. Rather, they result from the indirect influence of thyrotropin on thyroid function. Both forms are less common than primary dysfunction. An elevated level of thyrotropin in the blood may also indicate iodine deficiency. After removal of the thyroid gland due to cancer, care must also be taken to keep thyrotropin production low. This is because its mode of action of stimulating thyroid tissue to grow is not abolished even in the absence of a thyroid gland. As a result, malignant thyroid tissue can form, which in turn can give rise to carcinomas.