Objective: Subclinical hypothyroidism (SH) is a mild form of thyroid failure, associated with initial signs of cardiovascular hypothyroidism. Tissue Doppler echocardiography (TDE) is a new and powerful method in evaluation of both regional and global systolic or diastolic ventricular function. We aimed to investigate the use of TDE in evaluation of cardiac effects of SH and affect of thyroid hormone replacement therapy (TRT) on tissue Doppler parameters of SH patients. Methods: Twenty-two patients who were diagnosed as SH and 22 healthy, age and sex-matched cases were included in the study. Conventional echocardiography and TDE were performed in all individuals. TRT was started in SH group. On the achievement of euthyroid state echocardiography were repeated. Results: Septal annulus relaxation time was significantly higher in SH group (82 +/- 21, 98 +/- 11 ms, p=0.024). Lateral annulus and myocardial relaxation times, precontraction/contraction ratios and precontraction times were also slightly higher. Septal lateral annulus and lateral myocardial relaxation times were decreased after TRT (98 +/- 11 vs. 81 +/- 12, p < 0.001, 89 +/- 14 vs. 78 +/- 11, p=0.022, 90 +/- 16 vs. 80 +/- 14 ms, p=0.008, respectively). Precontraction times and precontraction/contraction ratios decreased after TRT but did not reach the significance level. There was a positive correlation between TSH and TDE relaxation times. Conclusions: TDE is a powerful tool in diagnosis and follow-up of SH patients and TRT inhibits adverse affects of SH on myocardium. Septal myocardium is the most affected region of left ventricle in SH. The relaxation time is the best criteria of cardiac involvement and monitoring the effect of TRT.