The role of plasma growth hormone, prolactin, triiodothyronine and tetraiodothyronine in the regulation of growth and sex differences in body weight of turkeys

An attempt was made to alter male:female (M:F) body weight ratios in two strains (S12 and S72) of turkeys by selective matings; to study their post-hatching growth patterns and to relate growth patterns to plasma growth hormone (GH), prolactin (Prl), tetraiodothyronine (T4), and triiodothyronine (T3) concentrations. Selection for high and low M:F body weight ratio (HR and LR) was essentially ineffective in both strains. Sex differences in body weight were first significant in S12 at day of age and in S72 at 6 wk. Plasma GH and Prl rose during the first 2 wk of age and fell thereafter while T3 levels decreased and T4 levels increased over time. Differences in plasma hormones between lines and sexes within strains were noted, but showed no consistent patterns. Correlations between GH and growth were mostly negative. When significant, regressions between body weight and GH or gain and GH were generally negative. Significant correlations between Prl and body weight in males were negative whereas they were of mixed sign in females. Correlations between T3 levels from 2 to 8 wk of age and body weight over this same period were mostly positive in males whereas those at later ages and in females were mostly negative. Few significant correlations between T4 and weight were found. These findings suggest that the regulation of growth in turkeys is quite different than that of mammals and that hormones known to regulate growth in some species may not do so in others.