Low efficiency of oxygen utilization during exercise in hyperthyroidism.
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 sub-maximal exercise.


METHODS
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 (delta Watt/delta VO2) was calculated as an index of work efficiency (where delta 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 +/- 28 x 10(2) 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, delta Watt/delta VO2 was significantly lower in hyperthyroid than euthyroid states (p < 0.001). There was a significant inverse correlation-ship between triiodothyronine and delta Watt/delta Vo2 (r = -0.654, p < 0.001).


CONCLUSIONS
Hyperthyroidism causes low work efficiency, which may limit exercise tolerance.
Department of Cardiology, Heart Institute of Japan, Tokyo Women's Medical College, Japan.
H
H
Kimura
Y
Y
Kawagoe
N
N
Kaneko
H E
HE
Fessler
S
S
Hosoda
D000328:Adult; D015308:Anaerobic Threshold; D013956:Antithyroid Agents / Q000627:therapeutic use; D001794:Blood Pressure; D005080:Exercise Test; D017079:Exercise Tolerance; D005260:Female; D005500:Follow-Up Studies; D006321:Heart / Q000503:physiopathology; D006339:Heart Rate; D006801:Humans; D006980:Hyperthyroidism / Q000188:drug therapy / Q000378:metabolism*; D008168:Lung / Q000503:physiopathology; D008297:Male; D008713:Methimazole / Q000627:therapeutic use; D008875:Middle Aged; D010101:Oxygen Consumption; D005082:Physical Exertion / Q000502:physiology*; D012146:Rest / Q000502:physiology; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D013956:Antithyroid Agents; D014284:Triiodothyronine; D008713:Methimazole; D013972:Thyrotropin; D013974:Thyroxine