Pituitary nuclear 3,5,3'-triiodothyronine and thyrotropin secretion: an explanation for the effect of thyroxine.

An excellent correlation was observed between nuclear triiodothyronine (T3) and the ensuing suppression of thyrotropin (TSH) after a single intravenous injection of T3 to thyroidectomized (hypothyroid) rats. At 1 and 2 hours after injection of thyroxine (T4), in amounts equally potent to the administered T3 in terms of acute suppression of TSH, the same quantities of T3 were found in the pituitary nuclei. Virtually no nuclear T4 was present, and plasma T3 was negligible at these short intervals after T4 injection. These results suggest that suppression of TSH release in hypothyroid rats occurs by interaction of T3 with the nuclear receptor of the thyrotroph. After T4 injection, the T3 found in the nucleus is derived from rapid intrapituitary monodeiodination.