Effects of dietary ascorbic acid, aspirin, lysine, and thiouracil on thyroid activity.

Broiler chickens were reared in batteries and fed diets designed to determine the effects of either ascorbic acid, aspirin (acetylsalicylic acid), lysine, or thiouracil on thyroid weight and serum thyroid hormone concentrations. Thyroxine (T4) and 3,5,5'-triiodothyronine (T3) concentrations in serum were determined by radioimmunoassay. Neither ascorbic acid nor lysine affected T3 or T4 concentration, but thiouracil significantly reduced T3 concentration after 1 day and reduced T4 concentration after 3 days. After 3 days or more of thiouracil feeding, relative reduction was greater for T4 than T3. Dietary aspirin significantly reduced T3 concentration at 7 of 16 sampling times but significantly reduced T4 concentration at only 1 of 16 sampling times. After 11 days of the dietary treatment, chickens fed thiouracil had significantly heavier thyroids than the controls but ascorbic acid, aspirin, and lysine had no effect on thyroid weight.