Impaired cardiac reserve and exercise capacity in patients on long-term thyrotropin suppressive therapy with levothyroxine.

To assess cardiac function and exercise tolerance in patients receiving long term TSH-suppressive therapy with levothyroxine (L-T-4), we studied maximal exercise capacity with a bicycle ergometer and left ventricular function at rest and during physical exercise by radionuclide angiography. The evaluation was performed in 10 patients receiving L-T-4 therapy (2.31+/-0.13 mu g/kg) for 5-9 yr, presenting with effort dyspnea and symptoms of adrenergic overactivity, and 10 matched control subjects. The patients were reassessed after 4 months of administration of the selective beta-adrenergic blocker bisoprolol (4.25+/-0.4 mg/day); L-T-4 therapy remained unchanged. The results showed that at rest, left ventricular diastolic filling was impaired in the patients (P <0.05), whereas systolic function was unaltered. During submaximal physical exercise, left ventricular ejection fraction increased in the controls from 58+/-2% to 65+/-2% (P <0.001), whereas in the patients it fell from 63+/-2% to 53+/-2% (P <0.01), mainly because of increased end-systolic left ventricular volume (P <0.05). Exercise capacity was markedly reduced in the patients in terms of both peak workload (P <0.001) and exercise duration (P