Triiodothyronine Reduces Growth Hormone Secretion and Pituitary Growth Hormone mRNA in the Chicken,in Vivo and in Vitro

Abstract

Abstract

The influence of triiodothyronine (T₃) on growth hormone (GH) mRNA and GH secretion has been examined in the chicken. Initially T₃ treatment in the diet for three days did not alter plasma concentrations of GH. Plasma concentrations of GH were depressed with seven and 14 days of T₃ treatment (1 or 5 ppm in the diet). There was a concomitant decline in pituitary GH mRNA with T₃ treatment. Pituitary GH content was reduced with 14 but not seven days of T₃ treatment. No effect of T₃ was observed on the percentage by somatotroph cells in the anterior pituitary gland by fluorescence flow cytometry analysis. While exposure of pituitary cells in vitro from young chickens to GHRF for 2 hr increased GH mRNA, no effect was observed with T₃. The presence of T₃, for 48 hr in vitro, tended to reduce GH mRNA in adenohypophyseal cells from young chickens and decreased GH mRNA with anterior pituitary cells from adult chicks. It is concluded that T₃ chronic administration of T₃ depresses circulating concentrations of GH, at least in part, by decreasing GH mRNA and hence GH synthesis.