“…Later on, experiments with injections of melatonin into laboratory animals demonstrated that this hormone exerts significant effects on various links of the functioning of the hypothalamo-hypophyseal-thyroid axis (HHTA) [23]. It was found that melatonin injection leads to a considerable decrease in the mass of the thyroid gland [21,22], suppresses the absorption of 131 I by thyroid parenchyma [24,25], disturbs intrathyroid hormonogenesis toward the predominance of iodotyrosine above iodothyronine [8,26], decreases the rate of secretion of thyroid hormones [10,27,28] and the index of free Т 4 [29], significantly decreases the concentration of thyroid hormones circulating in the blood, and also decreases the relative level of saturation of thyroid-binding globulin. Although species-and age-related peculiarities, diurnal or seasonal alterations of illumination in the given experiment, as well as the doses of melatonin used and pathways of its injections, were noticeably reflected on the data of examination of the above aspects of the effects of exogenous melatonin, the data obtained allowed researchers to qualify the epiphysis and its main hormone melatonin as factors responsible for inhibition of thyroid activity.…”