Tri-iodothyronine inhibition of thyrotrophin-releasing hormone-induced growth hormone release from the chicken adenohypophysis in vitro.

Tri-iodothyronine (T3) had no effect on the basal level of GH release from chicken hemipituitary glands perifused in vitro. The GH response to TRH was, however, markedly suppressed following exposure to T3. Suppression of TRH-stimulated GH secretion was observed after a 2-h preincubation with T3, and was induced, in a dose-related way, by 0.01-10 mumol T3/l. Exposure to T3 also reduced the effectiveness of TRh, at concentrations of 0.001-10 micrograms/ml, to stimulate GH release. These results demonstrate that, in addition to a hypothalamic site of action, T3 is likely to suppress GH secretion in vivo by direct effects on pituitary GH release.