Thyroid hormones increase liver and muscle thermogenic capacity in the little buntings (Emberiza pusilla)

Thyroid hormones can increase energy expenditure and stimulate basal thermogenesis by lowering metabolic efficiency. In the present study, we examined the effects of thyroid hormones on basal heat production as well as on several physiological and biochemical measures indicative of thermogenic capacity to test our hypothesis that thyroid hormones stimulate increases in thermogenesis in little buntings. Little buntings that fed on thyroxine (T4)–laced poultry food of 3 and 5 ppm concentrations showed increases in basal metabolic rate (BMR) during the 3-week acclimation. At the end, these buntings had lower body weights, higher levels of contents of mitochondrial protein, state 4 respiration and cytochrome c oxidase activity in liver and muscle, and higher concentrations of serum triiodothyronine (T3) and T4 compared to control buntings. These results support the argument that thyroid hormones play an important role in the regulation of thermogenic ability in buntings by stimulating mitochondrial respiration and enzyme activities associated with aerobic metabolism.