Skeletal muscle constitutes the major target organ for the thermogenic action of thyroid hormone. We examined the possible relation between energy expenditure (EE), thyroid status and the contents of Ca²⁺-ATPase and Na⁺,K⁺-ATPase in human skeletal muscle. Eleven hyperthyroid patients with Graves' disease were studied before and after medical treatment with methimazole and compared with eight healthy subjects. Muscle biopsies were taken from the vastus lateralis muscle and EE was determined by indirect calorimetry. Before treatment the patients had 2-5 fold elevated total plasma T₃ and 41% elevated EE compared to when euthyroidism had been achieved. In hyperthyroidism the content of Ca²⁺-ATPase was increased: (pmol/g, mean±SD) 6555±604 vs. 5212±1580 in euthyroidism (p=0.04) and 4523±1311 in healthy controls (p=0.0005). The content of Na⁺,K⁺-ATPase showed 89% increase in hyperthyroidism (pmol/g): 558±101 vs. 296±34 (p=0.0001) in euthyroidism and 278±52 pmol/g in healthy controls (p<0.0001). In euthyroidism the contents of both cation pumps did not differ from those of healthy controls. The Ca²⁺-ATPase content was significantly correlated to plasma T₃ and resting EE. This provides the first evidence that in human skeletal muscle, the capacity for Ca²⁺ recycling and active Na⁺,K⁺ transport are correlated to EE and thyroid status.