The Role of Plasma Thyroid Hormones in the Regulation of Body Weight of Single Comb White Leghorn and Broiler Embryos

Studies were conducted to determine if sex differences in embryo weights existed in a light-weight breed of chickens (the Single Comb White Leghorn, SCWL) and in a heavy broiler strain of chickens. Plasma thyroid hormone concentration was measured to determine its relationship to embryo weight. Male SCWL embryos were heavier than females at 15 and 17 days of incubation, but not at 13 days. Male broiler embryos were heavier than females at 13 and 15 days of incubation, but not at 17 days. There was no sex difference in the levels of plasma triiodothyronine (T3) or of tetraiodothyronine (T4) of SCWL embryos at those respective ages. The plasma T4 in male broiler embryos was significantly lower than that in the females at 15 and 17 days of incubation, but not at 13 days. There was no relationship between plasma T3, and the weight of SCWL embryos. There were significant linear regressions of embryo weight on the plasma T4 levels for SCWL males at 13 and 15 days of incubation and for SCWL females at 13 and 17 days of incubation, respectively. A significant regression of embryo weight on plasma T4 was found in female broiler embryos at 13 days of age, but not in females at other ages or in males at any of the ages studied.