Low efficiency of oxygen utilization during exercise in hyperthyroidism

Study objective: The mechanism of exercise intolerance in hyperthroidism has not been fully elucidated, This study was undertaken to determine if hyperthyroidism reduced the efficiency of submaximal exercise. Study design: We measured cardiorespiratory variables up to the anaerobic threshold (AT) during ramp-loading cycle ergometry in 12 patients (New York Heart Association functional class II or III). Studies were performed in the hyperthyroid state and repeated in the euthyroid state after 10 months of medical treatment. In 10-W steps from rest to the AT, we measured oxygen uptake (Vo(2)) as a measure of total body work rate, and pressure rate product (PRP) as a measure of cardiac work rate. Loading watts at AT divided by the increment of Vo(2) from rest to the AT (Delta Watt/Delta Vo(2)) was calculated as an index of work efficiency (where Delta means the increment of each value from rest to the AT). Results: Vo(2) and PRP at the AT were not significantly different between hyperthyroid and euthyroid states (Vo(2), 16.6 +/- 3.0 vs 17.5 +/- 2.3 mL/min/kg: PRP, 229 +/- 41 vs 28 +/- 28 x 10(2) mm Hg/min). However, lending watts at the AT were significantly lower in the hyperthyroid than the euthyroid state (28 +/- 22 vs 60 +/- 14 W: p < 0.01), Vo(2) and PRP while hyperthyroid were significantly higher than when euthyroid at every 10-W step during ramp-loading exercise. Furthermore, Delta Watt/Delta Vo(2) was significantly lower in hyperthyroid than euthyroid states (p < 0.001). There was a significant inverse correlationship between triiodothyronine and Delta Watt/Delta Vo(2) (r = -0.654, p < 0.001). Conclusion: Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.