Thyroid hormones and growth patterns of embryonic and posthatch chickens from lines selected for high and low juvenile body weight.

Lines of chickens divergently selected for high (HW) or low (LW) 56-day posthatch body weight were evaluated for growth, development of small intestine, liver and pectoral muscle, and plasma T3 and T4 concentrations. Measurements were taken at 11,17, and 20 days of incubation and 5, 12, 20, 31, and 61 days posthatch. HW chicks and their organs were consistently larger than LW chicks and their organs from day 17 of incubation onward, and followed growth patterns characteristic of these lines. Significant line by age interactions indicated that chickens divergently selected for juvenile body weight had different temporal patterns for plasma T3 and for T4 from 11 days of incubation to 61 days posthatch. There were no differences between males and females within each line for either T3 or T4. There were no simple relationships between the patterns of plasma concentrations of either T3 or T4 and the pattern of growth of the body or the organs studied.