Low Efficiency of Oxygen Utilization During Exercise in Hyperthyroidism

¹ From the Department of Cardiology, the Heart Institute of Japan, Tokyo Women's Medical College, Tokyo, Japan
² From the Division of Pulmonary and Critical Care Medicine, Department of Medicine, Johns Hopkins University School of Medicine, Baltimore

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 (VO₂) 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 VO₂ from rest to the AT (Watt/VO₂) was calculated as an index of work efficiency (where means the increment of each value from rest to the AT).

Results: VO₂ and PRP at the AT were not significantly different between hyperthyroid and euthyroid states (VO₂, 16.6±3.0 vs 17.5±2.3 mL/min/kg; PRP, 229±41 vs 218±28x10² 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). VO₂ and PRP while hyperthyroid were significantly higher than when euthyroid at every 10-W step during ramp-loading exercise. Furthermore, Watt/VO₂ was significantly lower in hyperthyroid than euthyroid states (p<0.001). There was a significant inverse correlationship between triiodothyronine and Watt/VO₂ (r=0.654, p<0.001).

Conclusion: Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.

Key Words: exercise capacity • hyperthyroidism • work efficiency

Submitted on January 19, 1996
Accepted on June 3, 1996

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