Searching the information on the involvement of the thyroid gland in metabolic processes in the organism of homoiothermal animals, it is possible to ascertain that mainly researchers dealt with the mechanisms of adaptation of the organism to the effects of cold and relatively few works studied the thyroid gland function under the influence of hyperthermia. Hyperthermia, as a factor of aggression accompanied by human activity, is often recorded among workers in hot shops of blast furnace and rolling metallurgical enterprises, glass factories, miners of deep coal mines, and also as a result of human presence in hot natural climatic conditions. The purpose of the study was to determine the dynamics of linear dimensions and volume of the thyroid gland in different periods of readaptation after the effect of chronic hyperthermia on the organism of experimental animals in comparison with the control group. Material and methods. The experimental study was carried out on 60 white linear male rats, which were 12 weeks old and weighed 180-230 g. During an experiment laboratory animals were contained in accordance with the rules, accepted by European convention on defence of the vertebrates used for an experiment and scientific aims (Strasbourg, 1986), in accordance with principles of Helsinki declaration, accepted General assembly of the World medical association (1964-2000). The animals of this series were divided into two groups, depending on the agents acting on them. The first group consisted of control rats, which were also in the chamber for 5 hours at the temperature of 21°C. The second group included animals which were subjected to chronic hyperthermia of moderate severity (42.0-43.1°C). Hyperthermia was simulated from 8 a.m. to 1 p.m. (5 hours daily) during 60 days. After sessions of hyperthermia for 1, 7, 15, 30 and 60 days, animals were decapitated from the experiment under ether anesthesia. Immediately after decapitation, the thyroid gland was removed together with the tracheo-laryngeal complex, and the thyroid gland dissection was performed. The thyroid gland organometry program included measuring the width and thickness of the right lobe using a caliper. Determination of the thyroid gland volume was carried out according to the formula V=(A×B×C×p/6)×100, where V is the volume of the lobe of the gland, A is the length, B is the width, and C is the thickness of lobe. The analysis of digital data was carried out using a computer program for organo- and morphometric studies. The results of the parameters of thyroid mass were processed with the help of statistical programs, the probability of error was less than 5% (p <0.05). Conclusions. We thoroughly studied the dynamics of thickness and width of rat thyroid lobes and also the volume after prolonged exposure to chronic hyperthermia. A significant decrease in the linear indexes of lobes was revealed in accordance with intact animals. Thus, the studied organometric parameters of mature rats such as thickness and width of the lobe, the volume of the thyroid gland developed under the influence of chronic hyperthermia of medium degree were lower than in intact animals at all times of observation. The maximum decrease occurred at the initial time of observation (1 and 7 days), and the minimum came later (60 days).
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