The Importance of Adrenal Hormones in the Implementation of Stress-Protective Effect of Low L-Thyroxine Doses

Although low L-thyroxine doses have previously been foundto increase body’s stress resistance, it has remained unclear whetherthis effect is related to the action of L-thyroxine itself or ismediated by its activating/potentiating impact on other hormones.The aim of this work was to study the role of adrenal hormones inthe implementation of the protective effect of low L-thyroxine dosesunder stress. The work was carried out on white mongrel male rats;stress was modeled by a time deficit situation. L-thyroxine wasadministered at low, close to physiological, doses according tothe following schedule: week 1—1.5 µg/kg, week 2—2.0 µg/kg, week3—2.5 µg/kg, week 4—3.0 µg/kg. It was found that stress exposure(1) increases the blood concentration of iodine-containing thyroidhormones (ITH) by 23–44%, corticosteroids by 40%, and adrenal relativemass (RM) by 31%, (2) reduces thymic and splenic RMs by 26 and 14%,respectively, and insulin level by 46%, (3) causes damage to thegastric mucosa (GM) with a damage index (DI) of 1.36, (4) increasesanimal physical endurance, locomotor activity and anxiety, whiledecreasing the intensity of exploratory responses. Bilateral adrenalectomy(BA) prevents the activation of thyroid function, abolishes theincrease in physical endurance, and alters the structure of behavior,as well as determines a more severe damage to the GM (DI 2.7), anincrease in anxiety, and the suppression of exploratory behaviorunder stress. BA limits the protective effect of L-thyroxine under stress,as manifested by lower values of rat horizontal locomotor and exploratoryactivity, higher anxiety, and damage to the GM (after L-thyroxineadministration, DI under stress without BA is 0.12, against BA is1.19). This corresponds to a lower blood ITH concentration (by 14–43%).Hence, low L-thyroxine doses have a stress-protective effect. BAlimits but does not completely prevents its implementation, suggestingthat adrenal hormones play an important, albeit not crucial, rolein shaping the L-thyroxine anti-stress effect.