Thyroid hormones in genetically lean or fat chickens: effects of age and triiodothyronine supplementation

Thyroid hormones were measured in plasma of genetically lean (LL) or fat (FL) chickens at different ages. No differences were observed at hatching or at adult age. More triiodothyronine (T3) and less thyroxine (T4) were found in the plasma of LL than in FL at the fed state during the growth period. This difference disappeared as the birds approached sexual maturity. Dietary supplementation by T3 increased the plasma concentration of T3 at the fed state. It did not influence growth rate, feed efficiency and body temperature. T3 supplementation tended to decrease abdominal fat proportion. It is suggested that the difference in plasma T3 would account for only a small proportion of the between-genotype differences in fattening.