It is well known that hyperthyroidism is one of the most frequent causes of atrial fibrillation. In hyperthyroid state, heart rate, and cardiac contractility are increased, which predisposes to atrial fibrillation. On the other hand, hypothyroidism decreases heart rate, and lowers myocardial contractility. Therefore, thyroid hormone defect could also result in significant changes in cardiac function. However, the cardiac effects of low thyroid hormone status have been less assessed so far. The aim of this thesis is to investigate the relationship between thyroid hormone deficiency and incidence of atrial fibrillation.