Background: Prolonged atrial conduction time measured by tissue Doppler imaging (TDI) has been associated with increased risk of atrial fibrillation. We aimed to evaluate the effect of subclinical hyperthyroidism (SH) and antithyroid treatment on atrial conduction time. Methods: A total of 30 patients with SH (26 females; mean age 34.8 +/- 8.5 years) and 30 age- and gender-matched controls were included. Using TDI, atrial conduction time was measured from the lateral mitral annulus, septal mitral annulus, and lateral tricuspid annulus. Intra- and interatrial conduction delay were calculated. TDI and thyroid hormone levels were studied at the time of enrollment and after achievement of euthyroid state with propylthiouracil treatment. Patients were followed for 14 +/- 3 weeks. Results: Atrial conduction time at the lateral and septal mitral annulus were significantly higher in patients with SH compared to controls. Both inter-, right, and left intraatrial electromechanical delay were prolonged in patients with SH compared to control subjects (21.3 +/- 6.1 vs. 13.9 +/- 4.3, P < 0.001 and 4.2 +/- 3.5 vs. 2.3 +/- 1.9, P = 0.014 and 17.1 +/- 6.0 vs. 11.6 +/- 3.8, P < 0.001, respectively). After achievement of euthyroid state, inter- and left intraatrial electromechanical delay were significantly decreased compared to baseline values and approximated to the values of the control group (P < 0.001). Conclusion: SH is associated with prolonged atrial conduction time. After achievement of euthyroid state, decrement in atrial conduction time may reveal how the antithyroid treatment may prevent the development of atrial fibrillation in these patients.