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

X. Tao^*,1, Z. Y. Zhang, H. Dong^*, H. Zhang and H. Xin

^* Institute of Agricultural Environment & Sustainable Development, National Key Laboratory of Animal Nutrition, Chinese Academy of Agricultural Sciences, 2 Yuan Ming Yuan West Road, Beijing 100094, China; and Department of Agricultural and Biosystems Engineering, 3204 National Swine Research and Information Center, Iowa State University, Ames 50011-3310

¹ Corresponding author: taoxp{at}cjac.org.cn

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.

Key Words: diurnal variation • broiler • thyroid hormone • heat stress • thermoneutral