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Background Atrial fibrillation is a common cardiac manifestation of hyperthyroidism. The relation between hyperthyroidism and atrial fibrillation has so far been analyzed in a limited number of selected patients, and the strength of the association has not been estimated. We examined the risk of atrial fibrillation among patients aged 20 to 89 years with hyperthyroidism diagnosed in hospitals in Denmark during a 20-year period. Methods We identified all patients with an incident hospital diagnosis of hyperthyroidism during the study period in the Danish National Registry of Patients, and among those we identified patients with a diagnosis of atrial fibrillation or flutter that occurred ±30 days from the date of the hospital diagnosis of hyperthyroidism. We used logistic regression analysis to calculate odds ratios (ORs) and 95% confidence intervals (CIs) for the association between sex, 10-year age group, cardiovascular diseases, and risk of atrial fibrillation or flutter. Results Among 40 628 patients diagnosed as having hyperthyroidism, 3362 (8.3%) were diagnosed as having atrial fibrillation or flutter within ±30 days from the date of the diagnosis of hyperthyroidism. The following factors were associated with risk of atrial fibrillation or flutter: male sex (OR, 1.8; 95% CI, 1.6-1.9), age (OR, 1.7; 95% CI, 1.7-1.8) per 10-year increment, ischemic heart disease (OR, 1.8; 95% CI, 1.6-2.0), congestive heart failure (OR, 3.9; 95% CI, 3.5-4.4), and heart valve disease (OR, 2.6; 95% CI, 1.9-3.4). Conclusion Male sex, increasing age, ischemic heart disease, congestive heart failure, and heart valve disease are associated with an increased risk of atrial fibrillation or flutter in patients with hyperthyroidism. [ABSTRACT FROM AUTHOR]