Effect of dietary thyroid hormone on growth and feed efficiency of broilers.

Broiler chickens were fed corn-soybean meal diets supplemented with either 3,5,3'-triiodothyronine (T3) or thyroxine (T4) in five trials. Body weight gain and feed efficiency are reported for four trials. Chickens fed T3 at 1.00 ppm had poorer weight gain and feed efficiency than controls, but T4 at that same dosage did not adversely affect performance. Serum T3, T4, and 3,3',5'-triiodothyronine (rT3) were assayed by radioimmunoassay. T3 or T4 at 1.00 ppm in the diet resulted in respective serum T3 and T4 concentrations approximately three times the control concentration. Dietary T4 at 10 ppm resulted in increased rT3 values after 1, 2, 5, and 6 days of T4 feeding but increased T3 values at 1 day only. T4 was apparently deiodinated to rT3 and toxic concentrations of T3 were not observed during T4 feeding. The data indicate that chickens respond differently to T3 than to T4.