Effect of dietary thyroid hormone on survival time during heat stress

Broilers were fed corn-soybean meal diets supplemented with either 3, 5, 3'-triiodothyronine (T3) or thyroxine (T4) for various time periods ranging from 8 to 26 days. Survival times during heat stress were determined for females and males fed the control, T3, and T4 diets. Serum hormone concentration for serums collected prior to heat stress was determined by radioimmunoassay. The combined results of both trials show significantly shortened survival times for T3 and T4 and that T3 had a much greater effect than T4. Male broilers had shorter survival times than females and there was no interaction of hormone and sex. Serum T3 and T4 concentrations were greater in serum T4 concentration, but dietary T4 had no effect on serum T3 concentration.