Effect of Thyroxine on the Weight and Composition of Muscle, Pelt and Other Tissues in Young Hypophysectomized Rats

Recently it was observed that the increase in weight of striated muscle in response to growth hormone administration was greater in hypophysectomized rats than in thyroidectomized rats (Scow, 1953). The difference in response between the two groups of animals was assumed to be due to the secretion of thyroid hormone, although diminished, in the hypophysectomized rat. The purpose of the present study was to determine if thyroxine given alone could effect a muscle growth response in hypophysectomized rats. Earlier, Ray et al. (1950) had reported a small increase in body weight and skeletal length of thyroxine-treated hypophysectomized rats. In the present experiments, not only were the findings of Ray et al. confirmed, but it was observed that the weight responses of skeletal and cardiac muscle, thymus, and pelt were proportionally greater than that of all other tissues. EXPERIMENTAL Male Holtzman rats at 21–23 days of age were hypophysectomized under sodium pentobarbital-ether anesthesia.