Production and metabolism of thyroid hormones in protein-deficient and food-restricted pregnant rats.

Thyroxine (T4) and triiodothyronine (T3) concentrations in the plasma and their metabolic clearance and production rates were studied in pregnant protein-deficient and food-restricted rats on the 20th day of gestation. Total T3 levels in the plasma were significantly reduced in malnourished dams, with those of food-restricted dams being consistently lower than those of protein-deficient dams. There were no changes in plasma total T4 concentrations as a result of dietary treatment. Unbound T4 and T3 in the plasma were significantly reduced in both protein-deficient and food-restricted dams. Maternal protein deficiency significantly lowered fetal T4, whereras food restriction caused an elevation. Metabolic clearance rates of T3 and T4 and production rate of T4 were unaffected by diet, while the production rate of T3 was significantly increased in protein deficient dams. Thyroxine to triiodothyronine ratios in malnourished dams were elevated suggesting depressed peripheral conversion of T4 to T3. Alternative factors which may lead to these effects are discussed.