Thyroid-vitamin A interactions in chicks exposed to 3,4,3',4'-tetrachlorobiphenyl: influence of low dietary vitamin A and iodine.

Poultry chicks receiving a low vitamin A semipurified diet and exposed to 3,4,3',4'-tetrachlorobiphenyl became hypothyroid in comparison with unexposed controls. Metabolic rate, total serum thyroxine, total serum triiodothyronine, and food intake decreased significantly while thyroid weight increased. Unexpectedly, growth rate was not affected on this diet. In the case of chicks receiving a low vitamin A--low iodine semipurified diet and exposed to the PCB congener, the hypothyroid response was apparently antagonized. Comparing exposed chicks with unexposed controls, metabolic rate and the proportion of free T3 (i.e., %T3 resin uptake) increased while total serum thyroxine and thyroid weight were unchanged. In addition, growth rate, food consumption, and serum retinol decreased on this diet. These results are interpreted to mean that growth rate may have been altered by circulating levels of retinol, and vitamin A insufficiency may predispose birds to the hypothyroid effects of PCBs.