OBJECTIVE: To investigate the effects of thyroid hormones on cardiac autonomic nervous activity and ventricular repolarization dynamicity in hyperthyroidism. METHODS: 57 consecutive patients first diagnosed of hyperthyroidism (HT group) and 55 age and sex-matched healthy volunteers (Control group) from March 2012 to March 2013 in our center were enrolled. All subjects underwent standard 12鈥憀ead ECG and 24鈥痟 Holter recording at baseline. For the HT group, free triiodothyronine (FT(3)), free thyroxine (FT(4)) and thyroid stimulating hormone (TSH) were monitored, and after they returned to normal all the examinations were redone. Heart rate variability (HRV) was assessed to determine the cardiac autonomic nervous activity. QTe/RR slope (QT end) and QTp/RR slope (QT apex) were calculated to evaluate the ventricular repolarization dynamicity. RESULTS: The HT patients before treatment had significantly higher LF/HF, QTe/RR slope and QTp/RR slope, and larger QT dispersion than the controls and after treatment (P鈥?鈥?.05 for all). Correlation analyses revealed that FT(3) was positively correlated with QTe/RR and QTp/RR slopes (r鈥?鈥?.689 and 0.665 respectively, P鈥?鈥?.001 for both), and similarly in FT(4) (r鈥?鈥?.665 and 0.668 respectively, P鈥?鈥?.001 for both). While TSH was negatively correlated with QTe/RR and QTp/RR slopes (r鈥?鈥?0.660 and -0.680 respectively, P鈥?鈥?.001 for both). FT(3) and FT(4) levels were independent predictors of QTe/RR slopes (P鈥?鈥?.001, beta鈥?鈥?.007; P鈥?鈥?.017, beta鈥?鈥?.001, respectively) and QTp/RR slopes (P鈥?鈥?.001, beta鈥?鈥?.008; P鈥?鈥?.002, beta鈥?鈥?.001, respectively). CONCLUSIONS: High-level thyroid hormones induce the cardiac sympathetic overactivity and increases ventricular repolarization dynamicity, and the impact can be attenuated after euthyroidism restored.