Peripheral markers of thyroid status unaffected by endurance training in rats

In order to more clearly define the relationship between endurance exercise training and thyroid activity we have measured T₄ turnover in exercise-trained and sedentary rats under conditions of controlled food intake and body weight. Rats were run on rodent treadmills at gradually increasing speeds and durations until at the end of 12 weeks they were running 2 h per day at 31 m/min. Sedentary rats were either allowed to eatad libitum or were food restricted so that they gained weight at the same rate as controls. Relative heart weight and plantaris citrate synthase activity were both significantly increased in trained animals compared to controls. The thyroxine secretion and degradation rates were not affected by the training. In addition, liver mitochondrial -glycerophosphate dehydrogenase and citrate synthase were measured as indicators of the thyroid state at the tissue level. Neither of these enzymes were increased in the trained rats. We conclude that increased thyroid activity is not essential for inducing and maintaining the adaptations in muscle mitochondrial enzymes which occur in response to prolonged endurance exercise training.