Effects of photoperiod on thyroid gland development and function in growing chicks: A biochemical and morphometric study

Context: Light treatment has a regulatory role in some growth-related functions, including thyroid development in chicks. Aims: This study aimed to investigate the effects of different photoperiod treatments on thyroid organ weight and serum thyroid hormone concentrations of broilers by use of biochemical and histological methods. Methods: After the hatching, 120 broiler chicks (Ross) were divided into two main groups according to sex. Both groups were then split into two sub-groups based on photoperiod treatment: 16 h (i.e. 16 h light:8 h dark) and 24 h (24 h light:0 h dark). Thyroid gland and blood samples of six animals from each group were obtained after slaughtering at 7-day intervals from Day 14 after hatching to Day 42. Serum concentrations of free triiodothyronine (FT3), free thyroxin (FT4) and thyroid-stimulating hormone (TSH) were determined by the chemiluminescence method for all groups. Thyroid weight, bodyweight and thyroid follicle diameter were also measured. Key results: Thyroid weight:bodyweight ratio generally started to increase from Day 14 to Day 42, with no significant (P > 0.05) difference among the groups at the same age. For both male and female broiler chicks, morphometric measures increased as birds grew. Serum FT3 and TSH concentrations slightly decreased and serum FT4 concentrations increased in growing chicks of both sexes. Conclusions: Extending the photoperiod from 16 to 24 h had no effects on thyroid gland development or functions in terms of both biochemical and morphometric parameters in broiler chicks. Implications: Continuous light has minimal effects on thyroid functions of growing broiler chicks to Day 42. © 2021 CSIRO.