Work capacity and oxygen uptake abnormalities in hyperthyroidism

The aim of our study was to evaluate the haemodynamic and the respiratory response to exercise in patients with hyperthyroidism before and 30 days after normalized thyroid hormones levels. These findings were compared with those of 10 control patients.

Thirty patients (23 women, aged 34.3 +/- 12 years) with untreated hyperthyroidism were studied. Twenty-four patients were treated with methimazole, 13 of which were also treated with propranolol. Six patients underwent surgery. A symptom-limited cardiopulmonary exercise test and an echocardiography were performed in all patients.

At rest patients with hyperthyroidism showed at echocardiography an increased cardiac index (P = 0.006 vs euthyroid, P = 0.007 vs normal) and a higher ejection fraction (P = 0.008 vs euthyroid, P = 0.007 vs normal). The duration of the exercise was lower in hyperthyroid patients (P = 0.006 vs euthyroid; P = 0.0068 vs normal). Anaerobic threshold was reached at 49.6% of peak VO2 during hyperthyroidism, at 60.8% during euthyroidism (P = 0.01) and at 62% in normal (P = 0.01). Work rate was lower in patients with hyperthyroidism at anaerobic threshold (P = 0.01 vs euthyroid, P = 0.03 vs normal) and at maximal work (P = 0.001 vs euthyroid, P = 0.01 vs normal). Patients in hyperthyroidism showed a lower increment of heart rate between rest and anaerobic threshold (P = 0.021 vs euthyroid, P < 0.0001 vs normal) and a lower VO2 at anaerobic threshold (P = 0.03 vs euthyroid; P = 0.04 vs normal). Oxygen pulse at anaerobic threshold was significantly reduced in hyperthyroidism (P = 0.04 vs euthyroid, P = 0.005 vs normal).

The mean result is that after only 30 days of appropriate antithyroid treatment there was an appreciable improvement of exertion capacity.