BACKGROUND: While adverse effects of overt hyperthyroidism on the cardiovascular system are well-known, the effects of subclinical hyperthyroidism are not clear. The aim of the study was to investigate the effects of short term mild L-thyroxine (LT4) suppression therapy on myocardial functions in a group of premenopausal women with goiter, by using echocardiographic methods and tissue Doppler imaging (TDI). METHODS: Sixteen participants with goiter received LT4 suppression therapy to keep TSH levels between 0.1-0.4 µIU/mL. After baseline and 1st month assessment, 6-weeks follow-up were scheduled until 6th month assessment to adjust the medication dose during study period. All TSH levels decreased below 0.4 µIU/mL by the end of first month and stayed below this level throughout study period. At the beginning of the study and at month 6, the thyroid ultrasonography, Holter monitorization test, stress test, electrocardiograms and echocardiograms of participants were assessed. This was followed by a comparison of baseline and 6th month data. RESULTS: Baseline and 6th month 2-D echocardiography measurements of participants revealed that mean left ventricle diameter in diastole (4.1±0.3 vs 3.8±0.2 mm) and posterior wall thickness in diastole (0.9±0.1 vs. 0.8±0.1 mm) decreased (P<0.05); while stroke volume (41.9±9.9 vs. 48±8.2), stroke volume index (25.6±5.4 vs. 29.4±4.7), cardiac output (3.5±1.4 vs. 3.9±0.9) and cardiac index (2.2±0.8 vs. 2.4±0.5) increased (P<0.05). Other 2D echocardiography parameters did not change significantly. The pulse wave Doppler examination, stress test and Holter monitorization of participants did not reveal any difference between baseline and 6th month measurements. No statistically significant difference was observed in measurements of TDI except decreased septum S velocity. CONCLUSIONS: Short term mild LT4 suppression treatment did not cause systolic or diastolic dysfunction, or conduction defect in the heart; therefore may be safe in premenopausal females with not known cardiac disease.