IMPAIRED CARDIAC RESERVE AND EXERCISE CAPACITY IN PATIENTS RECEIVING LONG-TERM THYROTROPIN SUPPRESSIVE THERAPY WITH LEVOTHYROXINE

To assess cardiac function and exercise tolerance in patients receivin g long term TSH-suppressive therapy with levothyroxine (L-T-4), we stu died maximal exercise capacity with a bicycle ergometer and left ventr icular function at rest and during physical exercise by radionuclide a ngiography. 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 dys pnea and symptoms of adrenergic overactivity, and 10 matched control s ubjects. The patients were reassessed after 4 months of administration of the selective beta-adrenergic blocker bisoprolol (4.25+/-0.4 mg/da y); 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 physi cal exercise, left ventricular ejection fraction increased in the cont rols from 58+/-2% to 65+/-2% (P <0.001), whereas in the patients it fe ll from 63+/-2% to 53+/-2% (P <0.01), mainly because of increased end- systolic left ventricular volume (P <0.05). Exercise capacity was mark edly reduced in the patients in terms of both peak workload (P <0.001) and exercise duration (P <0.001). beta-Adrenergic blockade prevented both the fall in ejection fraction and the increase in end-systolic vo lume during exercise, and improved exercise tolerance. In conclusion, our data show that long term TSH-suppressive therapy with L-T-4 is not as harmless as believed, because it may cause marked impairment of ca rdiac functional reserve and physical exercise capacity. Administratio n of a beta-blocking drug for 4 months caused significant improvement of cardiac performance and exercise tolerance.