Serum triiodothyronine in undernourished rats: dependence on dietary composition rather than total calorie or protein intake.

Serum triiodothyronine (T3) in rats did not change if food intake was restricted without altering the composition of the diet. Feeding a hypocaloric low-protein high-carbohydrate diet caused increases in serum T3, while feeding a hypocaloric high-protein low-carbohydrate diet caused decreases in serum T3. These changes were not related to reductions in intake of either calories or protein, and concomitant changes in serum thyroxine (T4) were not observed. Reduction in the dietary content of valine, without changes in the percentage of carbohydrate in the diet, caused increases in serum T3 when compared to growth-matched controls on a diet with normal valine concentration. Serum T3 in undernourished rats depends on the composition of the diet and not on total intake of calories or protein.