We report on a 46 year old patient with a history of paroxysmal atrial fibrillation who presented to our emergency room. Diagnostic evaluation showed elevated free peripheral thyroid hormone levels and thyrotropine (TSH) hormone within normal limits. Ultrasound of the thyroid was normal, and thyroid autoantibodies were found in the normal range. There was a positive family history for thyroid dysfunction. TSH-producing adenoma (TSHoma) of the pituitary gland - the main differential diagnosis - was excluded by cranial MRI and laboratory tests. Familial thyroid hormone resistance (Refetoff syndrome) was suspected and could be confirmed by detection of a pathogenic mutation within the beta-thyroidhormone receptor gene. After spontaneous conversion to sinusrhythm the patient was treated with a beta(1)-selective betareceptor blocker. Up to now, no specific treatment is available to correct the defective beta-thyroidhormone receptor.