Nevertheless, in order to maintain the serum T4 concentration at the higher T4 to T3 conversion, the thyroid gland is presumed to secrete more T4 to circulation. We cannot exclude the possibility that the activity of the hypothalamo–pituitary–thyroid gland (HPT) axis may probably be stimulated by in ovo leptin treatment. Leptin has been shown to influence the feedback of thyroid hormone on hypothalamic thyrotrophin-releasing hormone (TRH) secreting neurons (Wauters et al., 2000). However, as the present study focuses on hepatic deiodination of thyroid hormones, further studies are necessary to elaborate how the HPT axis is affected by in ovo leptin administration in the chicken.