Regulation of thyroid hormone metabolism during fasting and refeeding in chicken

Fasting and refeeding have considerable effects on thyroid hormone metaboli sm. In the present study, 8-day-old meat-type cockerels were subjected to a 2-day starvation period followed by 3 days' refeeding. Blood and tissue sa mples were collected at the start of the experiment, at 4, 24, and 48 h of starvation, and at 4, 8, 24, 48, and 72 h of refeeding. This study demonstr ates that in chicken, fasting decreased plasma T-3 and TSH levels and incre ased plasma T-4 concentrations. This was accompanied by increased hepatic t ype III deiodinase (D3) and decreased renal D3 activity. There were no chan ges in hepatic or renal type I deiodinase (D1). Refeeding restored normal p lasma T-3, T-4, and TSH levels, while hepatic D3 and renal D3 activities re turned to prefasting levels. Again hepatic D1 was not affected, but renal D 1 was lower than the ad libitum values during the entire refeeding period. These results confirm that liver D3 is involved in the regulation of plasma T-3 during fasting and refeeding in the chicken. Northern blot analysis de monstrated increased hepatic D3 mRNA levels during the first day of starvat ion that disappeared by the end of the second day; refeeding had no additio nal effects. These results suggest that in fasted chickens the rapid upregu lation of hepatic D3 occurs predominantly at a pretranslational level, wher eas the drop in hepatic D3 activity after refeeding is probably regulated a t a post-translational level. In addition, renal D3 may play a role in the regulation of local T-3 availability. (C) 1999 Academic Press.