Pituitary nuclear 3,5,3 '-triiodothyronine and thyrotropin secretion: An explanation for the effect of thyroxine (Reprinted from Science, vol 198, pg617-620, 1977)

An excellent correlation was observed between nuclear triiodothyronine (T-3 ) and the ensuing suppression of thyrotropin (TSH) after a single intraveno us injection of T-3 to thyroidectomized (hypothyroid) rats. At 1 and 2 hour s after injection of thyroxine (T-4), in amounts equally potent to the admi nistered T-3 in terms of acute suppression of TSH. the same quantities of T -3 were found in the pituitary nuclei. Virtually no nuclear T-4 was present , and plasma T-3 was negligible at these short intervals after T-4 injectio n. These results suggest that suppression of TSH release in hypothyroid rat s occurs by interaction of T-3 with the nuclear receptor of the thyrotroph. After T-4 injection, the T-3 found in the nucleus is derived from rapid in trapituitary monodeiodination.