Abstract The aim of the present study was to investigate in hypothyroid patients the reversibility of the prolongation of electromechanical delay and of the impairment of early diastolic relaxation as measured by radionuclide ventriculography after restoration of euthyroidism. We also evaluated Doppler echocardiography in relation to scintigraphic findings. Twenty-six consecutive hypothyroid patients were studied at diagnosis and after treatment. The time to peak ejection decreased (161 ± 6 vs 145 ± 5 ms; p = 0.03) without changes in global systolic function (ejection fraction). The peak filling rate was reversible as well: 2.6 ± 0.1 versus 3.0 ± 0.2 end-diastolic volume/s; p = 0.003. No significant changes in systolic and diastolic cardiac function or ventricular geometry were detected by Doppler echocardiography. However, subanalysis of profoundly hypothyroid patients (free T4 <4.5 pmol/L, n = 8) revealed a decrease in the septal wall thickness (9.9 ± 0.7 vs 8.3 ± 0.4 mm, p = 0.01) and increases in early peak transmitral inflow velocity (53 ± 4 vs 65 ± 4 cm/s, p = 0.03), as well as left ventricular end-diastolic diameter (4.8 ± 0.1 vs 5.1 ± 0.2 cm, p <0.05) after treatment. Thus, in primary hypothyroidism, a subtle reversible prolongation of contraction without major changes in global systolic function and a decrease in early active relaxation is observed by radionuclide ventriculography. Echocardiography shows changes only in severely hypothyroid patients.