Therefore, the objectives of the present study were to investigate the effect of in ovo leptin administration on hepatic expression and activity of iodothyronine deiodinases, in association with early posthatch growth rate and hepatic expression of GHR and IGF-I. Furthermore, chicken embryo hepatocytes (CEHs) were treated with leptin in vitro to reveal the direct effect of leptin on hepatic expression and activity of iodothyronine deiodinases.