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Nowadays a person in working conditions still has to face the adverse effects of high temperatures. In Ukraine, due to the unfavorable environmental situation, a further increase in thyroid function disorders is determined. Thus, the thyroid gland creates the prerequisites for the possible inclusion in the process of various mechanisms that can influence the change of its structure and function. The purpose of the study was to determine the morphological characteristics of the thyroid gland under the influence of prolonged overheating. The study was conducted on 60 laboratory adult white Wistar male rats with an initial mass of 180-230 g (10-12 weeks old). Exogenous hyperthermia was created using a thermal chamber. Group I consisted of control rats that were kept in a heat chamber for 5 hours at 21 °C, group II - animals exposed to chronic moderate hyperthermia (42.0-43.1 °C). Hyperthermia was modeled from 8 am to 1 pm (5 hours daily) for 60 days. After 60-day of hyperthermia sessions on 1-st day, 7-th, 15-th, 30-th and 60-th, the rehabilitation of animals was removed from the experiment by decapitation under ether narcosis. Thus, exploring the morphological changes, namely: an increase in the area of the follicles, an increase in the area of the colloid, an increase in the larger and smaller diameters of the follicles, a decrease in the height of the thyroid epithelium, a decrease in the area of the thyroid epithelium, a decrease in the thyrocyte count, a decrease in FKI and an increase in the INK, as well as a decrease in such morphometric indicators of thyrocyte nuclei as: a larger radius and smaller radius of the nucleus, as well as its area in all periods of the re-adaptation period in comparison with the control indicators can be made conclusions on the decrease in the functional activity of the thyroid gland.