The hypothalamic-pituitary-gonadal axis during hyperthyroidism in the rat.

The rat, an animal without testosterone-estrogen-binding-globulin, was treated with L-T4 to the point of hyperthyroidism in order to study the hypothalamic-pituitary-testicular axis during this condition. Hyperthyroidism led to a significant decline in serum FSH, a fall in serum LH which was not satistically significant, and no change in serum levels of testosterone or estradiol. Testes of hyperthyroid rats produced significantly more testosterone during in vitro incubations than did the testes of control animals. We conclude that hyperthyroidism in the rat leads to a fall in FSH levels either via direct pituitary suppression or via accelerated FSH metabolism. In addition, in vitro studies suggest that excess thyroid hormone may stimulate intratesticular 17 beta-hydroxysteroid dehydrogenase.