The Importance of Adrenal Hormones in the Implementation Stress-Protective Effect of Small Doses of L-thyroxine

Previously, the possibility of increasing the body's resistance to stress with small doses of L-thyroxine was shown. It has not been established whether this effect is related to the action of L-thyroxine itself or is mediated by its activation/potentiation of the effects of other hormones. The aim of the work is to study the role of adrenal hormones in the realization of the protective effect of low doses of L-thyroxine under stress. The work was performed on white mongrel male rats. Stress was modeled by a «time deficit» situation. L-thyroxine was administered in small, close to physiological doses according to the following scheme: the first week – 1.5 μg/kg, the second week – 2.0 μg/kg, the third week – 2.5 μg/kg, the fourth week – 3.0 μg/kg. It was found that stress exposure increases the concentration of iodine-containing thyroid hormones (ITH) in the blood by 23–44%, corticosteroids by 40%, the relative mass (RM) of the adrenal glands by 31%; reduces the RM of the thymus and spleen by 26 and 14%, the level of insulin by 46%; causes damage to the gastric mucosa (GM) with the damage index (DI) is 1.36; increases physical endurance, motor activity and anxiety of animals, reducing the intensity of research reactions. Bilateral adrenalectomy (BA) prevents the activation of thyroid function and eliminates the growth of physical endurance, changes the structure of behavior, determines more significant damage to the GM (DI is 2.7), increased anxiety, inhibition of research behavior under stress. BA limits the protective effect of L-thyroxine under stress, which is manifested by lower values of horizontal motor and research activity, higher anxiety in rats and damage to GM (after administration of L-thyroxine DI under stress without BA is 0.12; against BA is 1.19). This corresponds to a lower concentration of ITH in the blood (by 14–4 3%). Consequently, small doses of L-thyroxine have a stress-protective effect. BA limits, but does not completely prevent its implementation. This indicates that adrenal hormones play an important but not decisive role in the formation of the anti-stress effect of L-thyroxine.