Interactive effects of thermal environment and energy intake on thyroid hormone metabolism in newborn pigs

The aim of the study was to determine the effects of 48-hour cold exposure (25 vs. 34 degrees C), starting at 6 h of age, and energy intake on thyroid hormone plasma levels, hepatic 5'-deiodinase activity and nuclear 3,3',5-triiodothyronine (T3) receptors in newborn pigs. In the cold, piglets were fed either a similar milk intake (C piglets) to the piglets at thermal neutrality (TN piglets) or were given an extra amount of milk to compensate for the increased heat production (C+ piglets). Concentrations of plasma thyroxine (T4) decreased steadily over the 48-hour investigation and were affected neither by ambient temperature nor by energy intake. At the end of 48 h, plasma levels of T3 and liver 5'-deiodinase activity were similar in TN and C+ piglets and higher than in C piglets. The numbers of T3 receptors were greater in the liver from C and C+, as well as in skeletal muscle from C, than from TN piglets. Results of the study suggest that plasma T3, liver 5'-deiodinase and liver nuclear T3 receptors of the newborn pig can be regulated by energy intake and ambient temperature.