Responses of Thyroid Hormones of Market-Size Broilers to Thermoneutral Constant and Warm Cyclic Temperatures

This study characterizes the responses, particularly diurnal variations, of thyroid hormones (TH) [3,5,3'-triiodothyronine (T3), thyroxine (T4), and T3:T4 ratio (T3/T4)] of Arbor Acres broilers to constant thermoneutral (TN) or warm cyclic (WC) temperatures. There existed distinct circadian variations--2 peaks in TH under the TN and WC conditions. The 2 peaks of T3, T4, and T3/T4 of the broilers occurred, respectively, at 0 and 16 h, 8 and 16 h, and 0 and 12 h under the TN condition; but at 0 and 12 h, 0 and 8 h, and 4 and 12 h under the WC conditions. During a 5-d heat exposure (HE) to the WC regimens, T3 and T4 showed continual decrease. The daily mean of T3 declined significantly (P < 0.05) on the first day of HE, whereas significant decrease of T4 (P < 0.05) occurred on the second day of HE. There was no significant change in daily mean of T3/T4 during the same HE period. Hence, results of the study indicate that T3 provides a better heat stress indicator than T4. The nature of circadian variations in TH makes it necessary to collect blood samples more than once a day, e.g., 6 times at 0, 4, 8, 12, 16, and 20 h, to ensure full evaluation of hormonal responses of market-size broilers undergoing thermal challenges.