The present study demonstrated that leptin in ovo administration decreases hatch weight but improves early posthatch growth in broiler chickens. The expression and activity of hepatic iodothyronine deiodinases are altered towards enhanced thyroid hormone activation, which results in higher serum T3/T4 ratio in leptin-treated chickens. Significant up-regulation of hepatic GHR and IGF-1 mRNA expression is found to be associated with improved hapatic thyroid hormone activation and accelerated early posthatch growth in the chicken.