Influence of Thyroid Hormone on Diet-Induced Thermogenesis in the Rat

Energy balance and brown adipose tissue (BAT) metabolism were studied in rats maintained on stock or 'cafeteria' diet, and injected with either saline or triiodothyronine (T3, 10 micrograms/100 g b.wt./d) for 14 d. Cafeteria-fed rats showed large increases in metabolizable energy intake, energy expenditure and BAT mass, Na+, K+-ATpase activity and mitochondrial GDP binding. In stock fed rats, T3 also stimulated energy intake, metabolic rate and BAT mass and Na+, K+-ATPase activity, but did not affect GDP binding. Hyperthyroidism potentiated the effects of cafeteria feeding on energy expenditure and BAT mass, but BAT Na+, K+-ATPase activity was only slightly higher than that of the euthyroid cafeteria rats, and GDP binding was similar for both groups. These results confirm the involvement of BAT in diet-induced thermogenesis and show that this is potentiated by hyperthyroidism. The data also suggest that thyroid thermogenesis may result, at least partly, from stimulation of BAT.