MAINTENANCE OF THYROID-HORMONE PRODUCTION DURING EXERCISE-INDUCED WEIGHT-LOSS

Calorie restriction reduces thyroxine (T4) and 3,5,3'-triiodothyronine (T3) production, but the effects of exercise-induced weight loss on thyroid hormone metabolism in rodents are unclear. We studied the effects of chronic exercise on T4 and T3 metabolism comparing exercising (exercise) rats pair fed to sedentary (control) rodents and to weight-matched underfed sedentary animals (underfed; caloric intake 75% of ad libitum-fed controls). The exercise group utilized voluntary running wheels (28 days), and thyroid hormone metabolism was assessed using a three-compartment kinetics model. The exercise and underfed groups were equivalent in weight, but protein mass was greater in the exercise vs. underfed groups (30.4 +/- 0.5 vs. 27.9 +/- 0.5 g; P < 0.05). During exercise, the T4 plasma clearance rate (PCR) was decreased (-39.2%; P < 0.01) and the T4 concentration in serum was increased (48.6%; P < 0.01), resulting in an unchanged T4 plasma appearance rate (PAR) vs. the control group. The decrease in T4 PCR in the exercise group was associated with a lower transport rate of T4 out of the slow pool (P < 0.01). In the underfed group there was a reduction in both T4 serum concentration and PAR (-36%; P < 0.01) compared with the control group, which was associated with a decrease in the volume of distribution (-25%; P < 0.01). T3 PAR decreased 38.7% (P < 0.01) during underfeeding but only 16.9% (P = not significant) during exercise vs. the control group. The T3 PAR/T4 PAR ratio was similar in all three groups, indicating that exercise maintained T3 synthesis by maintaining T4 secretion. Tissue content of injected T4 was reduced 24 h postinjection during underfeeding, suggesting decreased T4 availability for conversion to T3. These findings indicate that exercise-induced weight loss is distinct from weight loss secondary to caloric restriction with respect to maintenance of T4 and T3 synthesis and protein balance.