In conclusion, the present findings extend our knowledge on the role of in ovo leptin in regulating the posthatch growth of chickens, via modulating hepatic expression/activity of iodothyronine deiodinases and hepatic expression of GHR and IGF-I. Enhanced thyroid hormone activation and higher serum T3/T4 ratio contribute to accelerated posthatch growth. Further studies are needed to elucidate the mechanisms underlying the sex-related responses and the signaling pathways by which leptin exerts its direct effect on deiodinases in cultured primary CEHs.