Effects of thyroxine supplementation on metabolic rate and aerobic capacity in a lizard

Effects of thyroxine (T4) on standard and maximal rates of O2 consumption and on citrate synthase activity, an index of mitochondrial oxidative capacity, were examined in the lizard Dipsosaurus dorsalis. Two weeks of daily injections of 200 ng T4/g body mass resulted in significant increases in standard (+60%) and maximal (+15%) rates of O2 consumption. Citrate synthase activity in liver and in red iliofibularis and gastrocnemius muscles was significantly higher than in controls. Correlational analyses indicate that intraspecific differences in standard metabolic rate may be partially attributable to fundamental differences in tissue metabolic capacities and that thyroxine injections lead to a general increase in aerobic capacities. Similar increases in aerobic capacities may occur in response to high thyroid activity during Spring in temperate-zone lizards. The injection protocol used in this study, taken from previous authors, results in suppression of natural thyroid secretory activity and in a 24-h average plasma T4 level of 95 ng/ml compared with 2.5 ng/ml in controls.