Effects of Thyroidectomy and Thyroxine on Hypothalamic Concentration of ‘Thyrotropin Releasing Factor’ and Pituitary Content of Thyrotropin in Rats

Abstract

When hypothalamic extract from mature male rats was incubated with anterior pituitary (AP) from male rats for 1 hr. at 37[degree]C in a Dubnoff metabolic shaker, a logdose relationship was observed between the concentration of hypothalamic extract used and quantity of thyrotropin (TSH) released by the AP. The effects of thyroidectomy for 30-45 days and of thyroxine (5, 10 or 25 [mu]g/100 g body weight) on hypothalamic concentration of "thyrotropin releasing factor" (TRF) and pituitary content of thyrotropin were studied in separate experiments. Hypothalamic extract from thyroidectomized rats elicited release of 2-3 times more TSH from incubated AP than hypothalamic extract from intact controls. The AP of thyroidectomized rats contained approximately 4 times as much TSH as the AP of intact rats. None of the doses of thyroxine injected altered TRF concentration of the hypothalamus, although TSH content of the AP was decreased to about 1/4 of control values. Pre-incubation of AP with thyroxine for 15 min., followed by addition of hypothalamic extract and incubation for 1 hr., inhibited hypothalamic stimulation of TSH release. These results suggest that thyroidectomy increases TSH secretion by the AP by enhancing hypothalamic synthesis and release of TRF, whereas thyroxine inhibits AP production of TSH by acting directly on the AP and preventing hypothalamic stimulation.