LOW EFFICIENCY OF OXYGEN UTILIZATION DURING EXERCISE IN HYPERTHYROIDISM

Study objective: The mechanism of exercise intolerance in hyperthroidi sm has not been fully elucidated, This study was undertaken to determi ne if hyperthyroidism reduced the efficiency of submaximal exercise. S tudy design: We measured cardiorespiratory variables up to the anaerob ic threshold (AT) during ramp-loading cycle ergometry in 12 patients ( New York Heart Association functional class II or III). Studies were p erformed 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 rat e. 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 eff iciency (where Delta means the increment of each value from rest to th e AT). Results: Vo(2) and PRP at the AT were not significantly differe nt between hyperthyroid and euthyroid states (Vo(2), 16.6 +/- 3.0 vs 1 7.5 +/- 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 hyp erthyroid than the euthyroid state (28 +/- 22 vs 60 +/- 14 W: p < 0.01 ), Vo(2) and PRP while hyperthyroid were significantly higher than whe n euthyroid at every 10-W step during ramp-loading exercise. Furthermo re, Delta Watt/Delta Vo(2) was significantly lower in hyperthyroid tha n euthyroid states (p < 0.001). There was a significant inverse correl ationship between triiodothyronine and Delta Watt/Delta Vo(2) (r = -0. 654, p < 0.001). Conclusion: Hyperthyroidism causes low work efficienc y, which may limit exercise tolerance.