Effects of thyroxine supplement on serum insulin-like growth factor-binding activity in propylthiouracil induced hypothyroid cockerels.

In propylthiouracil (PTU)-fed (g/kg feed) hypothyroid cockerels, serum levels of growth hormone (GH), but not insulin-like growth factor (IGF)-1, tended to rise and those of IGF-binding activity to fall. Thyroxine (T4) supplement (200 micrograms/kg/day) to PTU-fed cockerels for 8 days produced a hyperthyroid state and reversed these serum parameters. A chromatographic analysis of serum proteins revealed that T4 supplement markedly enhanced the IGF-binding activity of a 30 kDa protein and slightly lowered that of a 150 kDa protein, suggesting that T4 increases unsaturated IGF-binding proteins by reducing circulating IGF-1 concentrations.