Influence of thyroidectomy on serum and pituitary FSH in control and orchidectomized rats

The influence of hypothyroidism on both pituitary and serum FSH has been studied in male rats thyroidectomized in early stages of development. To know whether FSH alterations are produced by changes of the gonadal feed-back system or by a direct effect of thyroid hormones upon pituitary synthesis, estradiol, testosterone and testicular weight were measured. The result showed a decrease of biological and immunological activity of pituitary FSH in thyroidectomized rats. Serum FSH levels were also low. The deficiency of FSH production cannot be attributed to an alteration of the gonadal feed-back systems, since (1) in thyroidectomized rats, estradiol and testosterone were lower than in the controls, (2) signs of tubular atrophy were found in the testes and hence it could be expected that inhibin is low, and (3) in the absence of gonadal factors (induced by orchidectomy) thyroidectomized rats also showed a decrease of pituitary FSH. The present data suggest that the decrease of FSH can be attributed to a direct effect of thyroid hormones on pituitary FSH production.