Thyroid hormones and lipolysis in physically trained rats.

In rats a single bout of exercise resulted in increased triiodothyronine (T3), thyroxine (T4), and triiodothyronine/reverse triiodothyronine (T3/rT3) ratio 20 hr after exercise. The effect of norepinephrine on lipolysis in vitro was potentiated. In trained rats no changes were found in T4, T3, or rT3 concentrations. The T3/rT3 ratio as well as basal and stimulated TSH concentrations decreased in comparison with sedentary, freely eating rats. Moderate food restriction to produce a body weight similar to that of trained animals caused no changes in T4, T3, or rT3 concentrations but caused a decrease in T3/rT3 and in TSH levels. Training and moderate food restriction groups were not different. T3 in vitro caused a potentiation of catecholamine induced lipolysis in trained and food-restricted animals. With aging the serum concentration of T3 decreased and that of rT3 increased. Acute and chronic exercise both exert an effect on peripheral hormonal responses of lipolysis, while they have different and opposite effects on thyroid hormone concentrations. Physical training seems to have effects in this regard similar to those of moderate energy intake restriction. The results suggest that changes in peripheral effects of thyroid hormones during training should attract more attention.