Effect of cyclic heat stress on thyroidal hormones, thyroid histology, and performance of two broiler strains

This study determined thyroid hormone (T₃ and T₄) serum levels and investigated thyroid gland histology, as well as two broiler strains performance kept at different periods of heat stress. Broilers (560 Cobb Slow™ and 560 Hubbard Flex™) were distributed in control environment (0) reared following the thermal recommendations, and environments 1, 2, and 3 exposed to high temperatures for 1 h, 2 h, and 3 h a day, respectively, from 14 to 42 days old. At 21, 28, 35, and 42 days old, blood samples were collected to determine T₃ and T₄ serum levels. At 42 days old, thyroid glands were removed for histological analyses. The performance was calculated weekly. T₃ levels decreased with age advance, while T₄ serum concentration was higher at 42 days old (23.27 nmol L^-1) and lower at 28 days old (15.69 nmol L^-1). The histological sections revealed that thyroid morphology was well preserved. Live weight was higher in Cobb Slow™ (2.97 kg) than Hubbard Flex™ (2.93 kg) broilers at 42 days old. It is possible to conclude that cyclic heat stress exposure for a 3-h period from 14 to 42 days old did not affect performance, thyroid histology, or T₃ and T₄ serum levels. Cobb Slow™ broilers had the best live weight values at slaughtering. With increasing age, T₃ levels decreased and T₄ levels increased for both strains.