The low physical working capacity of thyrotoxic patients is not normalized by oral antithyroid treatment

The low working capacity in thyrotoxic patients has not been quantified and its causes have not been clarified. We studied the working capacity in nine thyrotoxic patients at diagnosis (test 1), after 3 months of oral antithyroid medication (test 2), and in four of them again after 1 year (test 3). The maximum power output was low in thyrotoxic patients (1.65 +/- 0.15 W/kg-1) and was not significantly higher after 3 months of treatment (1.84 +/- 0.15), although plasma thyroid hormone concentrations had been normalized for approximately 3 months. After 1 year's treatment the maximum power output was still somewhat low in the four tested patients (2.75 +/- 0.34). The rate of oxygen uptake under maximum exercise was low (26 ml/min-1/kg-1) and was not significantly increased at test 2 and 3. The net mechanical efficiency was low at test 1 and 2 (18.9 +/- 2.1% and 20.1 +/- 1.8%, respectively), but was normal at test 3 (25.2%). At all three tests the blood lactate response upon maximum exercise attained only about 50% of normal concentration. Thus, low work capacity in thyrotoxic patients remained for a long period of time after euthyroid conditions had been attained. The low capacity was due to low aerobic and anaerobic capacities and initially also to a low mechanical efficiency.