ALTERATIONS IN THYROID METABOLISM ARE ASSOCIATED WITH IMPROVED POSTHATCH GROWTH OF CHICKENS ADMINISTERED BOVINE GROWTH-HORMONE INOVO

Effects of in ovo administration of growth hormone (GH) on growth and thyroidal function of chickens were investigated in two experiments. I n 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 containin g 250 mug of pituitary bovine growth hormone (bGH) in trial 1 or conta ining 250 mug biosynthetic bGH in trial 2. In ovo administration of pi tuitary bGH but not biosynthetic bGH increased body weights and skelet al growth of male broilers at 3, 5, and 7 weeks posthatch. Seven-week- old males treated with pituitary bGH during embryogenesis exhibited de creased serum triiodothyronine (T3) levels and reduced liver-T4-5'-mon odeiodinase activity. Histological evaluation of thyroids from pituita ry bGH-treated broilers at 7 weeks posthatch indicated morphological a lterations consistent with depressed thyroid function, including reduc ed 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. Fert ile eggs were injected on day 11 of incubation with vehicle or 250 mug of pituitary bGH. At 5 weeks of age, serum T3 levels of broilers admi nistered pituitary bGH in ovo were significantly increased as compared to controls following a challenge with 0.25 mug TRH/kg body weight. C irculating T3 levels were increased in response to 2.5 mug TRH/kg body weight in both control and in ovo GH-treated broilers. In both experi ments, pituitary bGH administration resulted in significantly lower nu mbers of hatched chicks as compared to vehicle-injected chicks. Decrea sed hatchability, decreased circulating levels of T3, and increased se nsitivity to TRH are evidence consistent with thyroid hypofunction. Re duced metabolic rate associated with decreased thyroid metabolism may have resulted in greater availability of energy for anabolic processes such as growth.