The interaction between day length and the gonads in the regulation of levels of plasma thyroxine and triiodothyronine in the Japanese quail

Changes in the concentrations of plasma thyroxine (T₄) and triiodothyronine (T₃) were measured in intact and gonadectomized quail of both sexes after transfer from short (6L:18D) to long (18L:6D) days and upon return to short days. The concentration of free thyroxine (FT₄) was also measured in intact birds after transfer from short to long days. In both intact and gonadectomized birds, the concentration of plasma T₄ increased after transfer to long days while no consistent change was seen in the concentration of plasma T₃. However, after 15–24 days of photostimulation, the concentrations of plasma T₄ and T₃ began to fall in the intact birds but remained unchanged in the gonadectomized birds. The transitory increase in plasma T₄ observed in intact birds after photostimulation was associated with an increase in concentrations of plasma FT₄. The decrease in plasma T₄ and T₃ in photostimulated intact birds began when gonadal growth was nearly complete. The concentration of plasma T₃ in sexually mature male and female quail was lower than in sexually immature birds. In gonadectomized birds of both sexes, the concentration of plasma T₄ fell while that of plasma T₃ remained unchanged after transfer from long to short days. In sexually mature males and females, the transfer back to short days did not change the concentration of plasma T₄ but caused an increase in that of plasma T₃. These observations are consistent with the view that in the quail long day lengths may directly stimulate the hypothalamo-pituitary-thyroid axis and the production of T₄ and that the production of T₃ is not directly dependent on day length. It is suggested that in sexually mature birds the concentrations of plasma T₄ and T₃ are depressed because of a reduction in the concentration of plasma thyroid-hormone-binding proteins. The production of these proteins is suggested to be inhibited by gonadal steroids.