Peculiarities of circadian regulation of the thyroid gland

Circadian rhythmicity is an autonomous period of organ cells’ functioning that is controlled by feedback mechanisms and specific genes. The thyroid gland, as a peripheral organ of the endocrine system, is subordinate to the adenohypophysis and pineal gland. Melatonin, as one of the main mediators of circadian influence, is well known for its antitumor and regulatory effects on a number of tissues, but its relationship with the hormone-producing activity of the thyroid gland has not been sufficiently studied. The purpose of the work was to analyze the current data on circadian regulation of the thyroid gland by examining evidence-based scientific publications from open databases over the past five years. Melatonin-producing pineal cells possess the homeotic genes Otx2 and Crx whose deactivation leads to a significant decrease in melatonin production by the pineal gland. In addition to pineal cells, melatonin can be synthesized by adenohypophysis-independent parafollicular cells. The effect of melatonin on thyrocytes is provided by binding to MT1 receptors and affecting the expression of thyroglobulin, RX-8, and TTF-1 (NKX2-1) genes. Under conditions of short photoperiod, melatonin inhibits the production of thyroid-stimulating hormone (TSH) β, which, in turn, acts on hypothalamic tanycytes by regulating the balance of deiodinase (Dio2/Dio3). This leads to a well-regulated seasonal control of the release of the thyroid hormone triiodothyronine. Conclusions. Circadian regulation of thyrocyte activity occurs under the influence of melatonin, which is secreted by pinealocytes and sometimes by parafollicular cells. TSH is secreted with a certain circadian periodicity under the influence of the activation of chronorhythmic genes. Hormone replacement therapy should be aimed not only at restoring the levels of pituitary hormones, but also at restoring the biological rhythm of TSH secretion. Melatonin affects the circadian work of thyrocytes, which is proven by determining the expression of the chronorhythm genes Bmal-1, Dio2, TTF-1. In thyroid pathology, there is a violation of circadian rhythms, a decrease in melatonin levels, and a decrease in the expression of Clock genes.