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Temperature has unequivocal effects on several aspects of fish physiology, but the full extent of its interaction with key endocrine signaling systems to influence metabolic function remains unknown. The aim of the current study was to assess the individual and combined effects of elevated temperature and hyperthyroidism on hepatic metabolism in juvenile lake whitefish by quantifying mRNA abundance and activity of key metabolic enzymes. Fish were exposed to 13 (control), 17 or 21 degrees C for 0, 4, 8 or 24 days in the presence or absence of low-T-4 (1 mu g x g body weight(-1)) or high-T-4 (10 mu g x g body weight(-1)) treatment. Our results demonstrate moderate sensitivity to elevated temperature in this species, characterized by short-term changes in mRNA abundance of several metabolic enzymes and long-term declines in citrate synthase (CS) and cytochrome c oxidase (COX) activities. T-4 -induced hyperthyroidism also had several short-term effects on mRNA abundance of metabolic transcripts, including depressions in acetyl-coA carboxylase beta (acc beta) and carnitine palmitoyltransferase 1 beta (cpt1 beta), and stabilization of cs mRNA levels; however, these effects were primarily limited to elevated temperature groups, indicating temperature-dependent effects of exogenous T-4 treatment in this species. In contrast, maximal CS and COX activities were not altered by hyperthyroidism at any temperature. Collectively, our data suggest that temperature has the potential to manipulate thyroid hormone physiology in juvenile lake whitefish and, under warm-conditions, hyperthyroidism may suppress certain elements of the beta-oxidation pathway without substantial impacts on overall cellular oxidative capacity.