Effect of thyroid hormones on growth hormone secretion in broiler chickens.

Plasma concentrations of growth hormone (GH) were observed to be significantly elevated following the administration of thyrotropin-releasing hormone (TRH) in young (4- to 12-week-old) but not in adult (20-week-old) broiler chickens. However, adult sex-linked (dw) dwarf hens did respond to TRH. Treatment with triiodothyronine (T3) but not thyroxine (T4) (at 1 ppm in the diet from hatch) consistently and significantly reduced the growth rate and decreased the plasma concentrations of GH following TRH injection in normal (DwDw males or Dw-females), hemizygous dwarf (dw-) female, and heterozygous (Dwdw) male lines of broiler chickens. Similarly, T3 was significantly more effective than T4 in inhibiting the increase in plasma concentrations of GH following TRH injection in broiler chicks made hypothyroid by methimazole administration.