Differential effects of T4 and T3 on TRH- and GRF-induced GH secretion in the domestic fowl.

The in vivo growth hormone (GH) response of immature domestic fowl to thyrotrophin-releasing hormone (TRH) and GH-releasing factor (GRF) was suppressed in birds fed diets supplemented (1 ppm) with triiodothyronine (T3) or given bolus intraperitoneal (ip) injections (100 micrograms/kg for 10 d) of T3. Supplementation (ppm) of the diet with T4 had no effect on secretagogue- induced GH release. Exogenous T3 or T4 suppressed basal, TRH- and GRF-induced GH release 2 h after daily ip administration (100 micrograms/kg for 10 d). 24 h after the last injection, only T3 was effective in inhibiting basal and stimulated GH secretion in vivo. The systemic administration of T3 was followed 2 h and 24 h later by a downregulation of pituitary TRH binding sites. T4 administration had no effect on pituitary TRH binding. When chicken pituitary glands were incubated in vitro, basal GH release was unaffected by the addition of 10(-9)-10(-5) M T3 or T4 to the incubation media. The in vitro GH response to TRH (10(-6) M) or GRF (10(-6) M) challenge was, however, suppressed in a dose-related manner by T3 but was unaffected by the coincubation of T4. These results demonstrate inhibitory effects of T3 and T4 on basal and secretagogue-induced GH secretion in fowl. T4 is less active than T3 and probably exerts some of its effects via T3-independent mechanisms.