Plasma concentrations of thyroid hormones in growing pullets and laying hens reared under two tropical climates and fed on diets with different energy concentrations

Abstract

1. Blood samples were taken from growing (14-week-old) and laying (36-week-old) hens reared in either a temperate or a hot, humid climate and fed on 2 dietary energy concentrations (10.03 or 11.7 MJ ME/kg) to determine the plasma concentrations of 3,3 ,5-triiodothyronine (T3) and thyroxine (T4) by radioimmunoassay. 2. Plasma T3 concentrations were higher in birds fed on the low energy diet and higher at 14 than at 36 weeks of age, while the effects of climate were complex. 3. Plasma T4 concentrations were higher at 14 than at 36 weeks of age and interactions with the dietary and climatic factors were observed. 4. Interactions between dietary and climatic factors were observed since plasma T3 and T4 concentrations were higher at 14 weeks under the hot, humid than under the temperate climate. Furthermore, at 36 weeks plasma T3 concentrations were higher under the temperate climate and plasma T4 concentrations did not differ. 5. These findings are not easy to explain. However it may be suggested that the energy content of the diet clearly affects the plasma concentrations of thyroid hormones and that this effect is modulated by the climate to a variable degree that is linked to the age and the physiological state of the birds.