The aim of the present study was to evaluate the effect of subclinical hyperthyroidism (SHT) on cardiovascular autonomic function and ventricular repolarization. METHODS: Thirty subjects (25 females; mean age 49.6 +/- 9.8 years) with SHT, as judged by reduced TSH serum levels and normal free T4 and T3 serum levels, and 30 age and sex-matched control subjects underwent standard 12-lead ECG, and 24h ambulatory ECG monitoring. The dispersion of the QT interval, an index of inhomogeneity of repolarization, and the heart rate variability (HRV), a measure of cardiac autonomic modulation, were studied. RESULTS: Patients with SHT showed higher QT dispersion (p<0.001) and lower HRV measures (0.01>p<0.001) than controls. In SHT patients, QT dispersion was inversely related to HRV (r=-0.47, p<0.01). CONCLUSION: The results of the present study demonstrated that SHT is associated with a sympathovagal imbalance, characterized by increased sympathetic activity in the presence of diminished vagal tone, and with an increased inhomogeneity of ventricular recovery times. The assessment of HRV and QT dispersion in patients with SHT may represent a useful tool in monitoring the cardiovascular risk of this condition.