Thyroid function in laying, incubating, and broody bantam hens

Abstract

The concentration of plasma triiodothyronine (T 3) increased in broody bantam hens within 1 or 2 days of the onset of incubation and rose further after the chicks hatched. In a separate study, blood samples were taken from hens over a 24-hr period (14L:10D) when they were laying, incubating, or brooding chicks. The concentrations of plasma T 3 tended to drop during the dark period in laying, incubating, and brooding birds, while the converse changes were observed in the concentration of plasma thyroxine (T 4). A daily rhythm in the levels of plasma T 4 was not observed in incubating bantams. The possibility that prolactin is involved in the regulation of thyroid function during broodiness was investigated by measuring the concentrations of plasma thyroid hormones after an injection of bovine prolactin. The concentration of plasma T 4 was not affected by the injection of prolactin, whereas the concentration of T 3 in laying and incubating bantam hens was significantly increased after 60 min. This study suggests that one of the factors which influences thyroid function in broody bantams may be the increase in prolactin secretion.