To evaluate the effects of the acute loss of thyroid hormones on the heart the authors studied 11 women with acute hypothyroidism. The cardiac study was performed by means of electrocardiography (ECG), radionuclide ventriculography (RNV) at rest and under effort, and monodimensional echocardiography (MD-echo) and was repeated with ECG and MD-echo after six months of thyroxine suppressive therapy. The ECG showed a significant prolongation of QT interval and flattening and inversion of T wave with normal heart rate. The MD-echo revealed left ventricular function in the normal range and normal left ventricular dimensions. RNV showed the ejection fraction in the lower normal range at rest, which increased to a smaller extent under effort in comparison with a control group. The ECG performed during suppressive therapy with L-thyroxine yielded normal findings with an increase of heart rate and of R wave amplitude, and the MD-echo showed no significant variations of cardiac function due to the increase of heart rate.