Metabolic and hormonal effects of feeding chickens thyroxine and diets containing varied calorie to protein ratios

Indian River male broiler chickens, growing from 7 to 28 d of age, were fed diets containing energy to protein ratios from 43 to 106 MJ/kg of protein + 0 or 1 mg/kg thyroxine (T₄) to study both the energetic and metabolic costs of lipogenesis. De novo lipid synthesis was determined in the presence and absence of ouabain (ATPase inhibitor) to estimate the energetic costs of ion pumping in explants synthesizing lipid. Growth and feed consumption increased (P<0.05) when the energy to protein ratio decreased from 106 to 71 MJ/kg protein; but, both parameters decreased as the ratio was further decreased from 53 to 43 MJ/kg of protein. A large calorie-to-protein ratio and dietary T₄ depressed plasma growth hormone and insulin-like growth factor-I. A large calorie-to-protein ratio increased (P<0.05) lipogenesis and the cellular energy expended in support of Na⁺/K⁺ transport. In addition, plasma thyroid hormone ratios were changed by both energy to protein ratios and T₄.