Study objective: The mechanism of exercise intolerance in hyperthyroidism 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 (V̇o2) 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 V̇o2 from rest to the AT (ΔWatt/ΔV̇o2) was calculated as an index of work efficiency (where Δ means the increment of each value from rest to the AT). Results: V̇o2 and PRP at the AT were not significantly different between hyperthyroid and euthyroid states (V̇o2, 16.6±3.0 vs 17.5±2.3 mL/min/kg; PRP, 229±41 vs 218±28 x 102 mm Hg/min). However, loading watts at the AT were significantly lower in the hyperthyroid than the euthyroid state (28±22 vs 60±14 W: p<0.01). V̇o2 and PRP while hyperthyroid were significantly higher than when euthyroid at every 10-W step during ramp-loading exercise. Furthermore, ΔWatt/ΔV̇o2 was significantly lower in hyperthyroid than euthyroid states (p<0.001). There was a significant inverse correlation ship between triiodothyronine and ΔWatt/ΔV̇o2 (r=-0.654, p<0.001). Conclusion: Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.
- exercise capacity
- work efficiency
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine
- Critical Care and Intensive Care Medicine
- Cardiology and Cardiovascular Medicine