Effect of physical training on the metabolism of thyroid hormones in man

The effect of a 6-wk program of physical training (track running) on the peripheral metabolism of thyroxine (T4) and triiodothyronine (T3) was evaluated in a group of 11 men. Measurements were made of hormone turnover, urinay and fecal clearances, plasma hormone concentrations, and hormone binding by plasma proteins in all subjects before and after training, After training, metabolic clearance of T3 was increased 8.5% above the pretraining level due to an increased deiodinative clearance of this hormone. No significant change was observed in plasma T3 concentration. The absolute degradation of T3 increased 10.3% after training. In contrast, no significant change in the metabolic clearance of T4 was detected. Significantly decreased plasma concentration of total T4 after 4 and 6 wk of training was apparently not due to decreased hormone binding by plasma protein since no significant alteration in the dialyzable fraction of T4 was detected. The absolute degradation rate of T4 was decreased 8.8% after training. Possible implications of the observed differential impact of training on the degradation of T4 and T3 regarding thyroid hormone economy are discussed.