Thyroid and adrenal response to heat stress in chickens and quail differing in heat tolerance.

Three experiments were conducted to evaluate the thyroid and adrenal response in groups of birds with altered heat tolerance. Groups of chickens that had been handled on each of 4 days and a nonhandled control group were bled on the 5th day with or without heating for 1 hr at 50 C. Handling did not affect the thyroid response as indicated by thyroxine (T4) or triiodothyronine (T3) levels. Corticosterone levels were lower in handled birds than nonhandled birds after heating. In a second experiment, groups of broilers were fed a diet containing .2% thiouracil or a control diet. After 2 weeks they were bled either with or without 1 hr of heating at 50 C. The T3 and T4 levels were reduced by thiouracil feeding both with and without heating, but adrenal function was not affected. In Experiment 3, T3 and T4 levels were compared for four lines of Japanese quail. The nongrowth-selected line had higher T3 and T4 levels than growth-selected lines but did not have elevated levels of T3 or T4 in response to heat stress. The growth-selected lines (T, S, and P) responded to heating with increased T3 and T4 levels. Levels of T4 in Line T (selected on a thiouracil diet) after heating were significantly higher than all other lines.