Atrial fibrillation may occur in patients with a variety of cardiovascular or chronic disease as well as in normal subjects. Many authors reported that atrial fibrillation occurs in patients with thyrotoxicosis. It is reported that a low serum thyrotrophin concentration in an asymptomatic person with normal serum thyroid hormone concentrations can be a independent risk factor for developing atrial fibrillation. But we focused on the significance of serum thyroid stimulating hormone (TSH) in the euthyroid patient with atrial fibrillation whose serum level of T3, T4, fT4, and even TSH were absolutely within normal range. On our results, there was no significant differences in age, sexual distribution, and left ventricular ejection fraction between the patients group of paroxysmal and chronic persistent atrial fibrillation (p> 0.05), but there was larger left atrial dimension (LAD) and more cases of rheumatic heart disease in the chronic persistent atrial fibrillation group and there was more cases of lone atrial fibrillation in the paroxysmal atrial fibrillation group (p< 0.05). There was no significant differences in serum levels of T3, T4, fT4 between paroxysmal and chronic persistent atrial fibrillation, but significantly lower serum TSH was found in patients with paroxysmal atrial fibrillation (p< 0.001), and these findings were more significant after the control of hemodynamic change (p< 0.001 vs p< 0.05). The discriminant value in serum TSH between the paroxysmal and chronic atrial fibrillation group was 1.568U/mL with about 76% of predictive power. There was significantly lower serum TSH in paroxysmal atrial fibrillation in all age groups (p< 0.05). There was a significantly higher prevalence of cerebral thromboembolic events in chronic persistent (27.7%) and disease-associated (15.0% atrial fibrillation than in the paroxysmal (3.3%) and lone (4.5%) atrial fibrillation group (p< 0.001). Therefore, we suggest that serum TSH below the serum concentration of 1.5U/mL can be a risk factor for developing atrial fibrillation when the serum level of T3, T4, fT4, and even TSH were within absolutely normal range.