The thyroid gland produces the hormones thyroxine and triiodothyronine. Both hormones have a similar multifaceted effect on the body, increase tissue oxygen demand, enhance energy processes, stimulate tissue growth and differentiation, affect the functional state of the nervous and cardiovascular systems, liver, kidneys and other organs, enhance glucose absorption and utilization. The effect of thyroid hormones can vary depending on the dose. So, small doses of thyroxine have an anabolic effect, while large doses lead to increased protein breakdown. In high doses, thyroid hormones inhibit the thyroid-stimulating activity of the pituitary gland by a feedback mechanism.
Triiodothyronine is 3-5 times more active than thyroxine and acts faster, since it binds less to blood proteins (circulates in the blood mainly in free form) and penetrates cell membranes faster. The latent period of action of triiodothyronine is 4–8 hours, and thyroxine is 24–48 hours. In medical practice, synthetic analogs of thyroxine (sodium levothyroxine) and triiodothyronine (liothyronine) are used. Prescribe levothyroxine sodium and liothyronine (in relatively small doses) with insufficient thyroid function and in higher doses (not causing hyperthyroidism, but sufficient to suppress thyroid-stimulating activity) with excessive thyroid-stimulating function of the pituitary gland. To compensate for iodine deficiency, these drugs often include potassium iodide or prescribe it additionally. Combination preparations containing thyroid hormones and, in some cases, potassium iodide supplements are also used.
Thyroid hormone antagonists are antithyroid drugs.
Overproduction of the thyroid hormone – thyroxine leads to the development of serious diseases (thyrotoxicosis, Graves disease, thyrotoxic goiter), which are treated with the help of pharmacotherapy, surgery or their combination.
Antithyroid drugs (pharmacotherapy) are widely used to treat hyperthyroidism. One of the main synthetic agents in this group is thiamazole; this also includes propylthiouracil and some other drugs. The mechanism of their action is associated with a decrease in the iodination of thyroxine in the thyroid gland by blocking the inclusion of an iodine atom in the thyroxine molecule. They accelerate the excretion of iodides from the thyroid gland, inhibit the activity of enzyme systems involved in the oxidation of iodides with inhibition of iodination of thyroglobulin and delay in the conversion of diiodotyrosine into tri- and tetraiodotyrosine, or with inhibition of the release of thyroid-stimulating hormone from the pituitary gland.
The thyroid gland also produces calcitonin, a hormone of hypocalcemic action. Currently, 8 types of calcitonin are known, of which salmon, pig and human calcitonin are of medical importance. Calcitonin is produced by parafollicular cells of the thyroid gland, as well as cells of the thymus and parathyroid glands. A technology for obtaining synthetic calcitonin has also been developed.
Below is a list of thyroid hormones, their analogues and antagonists:
- Levothyroxine sodium
- Potassium iodide (Kalii iodidum)