Partial food restriction increases hepatic inner ring deiodinating activity in the chicken and the rat

The effects of a long term partial food restriction were studied in chickens and rats. In chickens the treatment resulted in increased plasma T4 levels while T4 levels in rats remained unchanged. Plasma T3 decreased in both species. In vitro hepatic outer ring deiodinating type I (ORD-I) activity was not influenced by the food restriction, suggesting that the amount of ORD-I enzyme present in the liver remained unchanged. In vitro hepatic inner ring deiodinating type III activity, on the contrary, was greatly increased in both species. This increase may contribute to the decreased circulating T3 levels by increasing the degradation of T3 and diverting the deiodination of T4 to rT3 instead of to T3.