Alterations in thyroid metabolism are associated with improved posthatch growth of chickens administered bovine growth hormone in ovo.

Effects of in ovo administration of growth hormone (GH) on growth and thyroidal function of chickens were investigated in two experiments. In Experiment 1, fertile eggs were injected on day 11 of embryogenesis with vehicle (0.03 M NaHCO3, 0.15 M NaCl, pH 8.3) or vehicle containing 250 micrograms of pituitary bovine growth hormone (bGH) in trial 1 or containing 250 micrograms biosynthetic bGH in trial 2. In ovo administration of pituitary bGH but not biosynthetic bGH increased body weights and skeletal growth of male broilers at 3, 5, and 7 weeks posthatch. Seven-week-old males treated with pituitary bGH during embryogenesis exhibited decreased serum triiodothyronine (T3) levels and reduced liver-T4-5'-monodeiodinase activity. Histological evaluation of thyroids from pituitary bGH-treated broilers at 7 weeks posthatch indicated morphological alterations consistent with depressed thyroid function, including reduced amounts of non-follicular tissue and increased mean follicular area. A second experiment was initiated to further investigate the effects of In ovo administration of pituitary bGH on thyroid metabolism. Fertile eggs were injected on day 11 of incubation with vehicle or 250 micrograms of pituitary bGH. At 5 weeks of age, serum T3 levels of broilers administered pituitary bGH in ovo were significantly increased as compared to controls following a challenge with 0.25 micrograms TRH/kg body weight. Circulating T3 levels were increased in response to 2.5 micrograms TRH/kg body weight in both control and in ovo GH-treated broilers. In both experiments, pituitary bGH administration resulted in significantly lower numbers of hatched chicks as compared to vehicle-injected chicks. Decreased hatchability, decreased circulating levels of T3, and increased sensitivity to TRH are evidence consistent with thyroid hypofunction. Reduced metabolic rate associated with decreased thyroid metabolism may have resulted in greater availability of energy for anabolic processes such as growth.