Sex steroids modulate rat anterior pituitary and liver iodothyronine deiodinase activities

In this study, we investigated the sex hormone regulation of 5'-iodothyroni ne deiodinase activity, which is responsible for enzymatic conversion of th yroxine into the bioactive form, triiodothyronine. Pituitary homogenates an d liver microsomes from: 1) ovariectomized rats injected with 17-beta -estr adiol benzoate and/or progesterone (0.7 and 250 mug/100 g body weight, resp ectively, subcutaneously, over 10 days); 2) male castrated rats treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular, o ver 7 days, were assayed for type I and type 2 deiodinase activity in the p ituitary. Enzyme activities were measured by release of I-125 from deiodina tion of I-125 reverse triiodothyronine under varying assay conditions. Estr ogen stimulated anterior pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30%, p < 0.05). Progesterone inhibited the live r enzyme (40%, p<0.05), and had no effect on the pituitary, but in both tis sues, blocked estrogen stimulatory effect on type 1 deiodinase. In males, t estosterone normalized the reduced liver type 1 deiodinase of castrated rat s. However, in the pituitary, castration increased (50%) type 1 deiodinase independent of testosterone treatment, suggesting the existence of a inhibi tory testicular regulator of pituitary type 1 enzyme. Treatments did not al ter pituitary type 2 deiodinase activity. In conclusion, gonads and sex ste roids differentially modulate type 1 deiodinase activity in rat pituitary a nd liver.