Amiodarone has long been the anti-arrhythmic drug of choice for the treatment of atrial fibrillation. We use a clinical case report to illustrate the difficulty of treating this arrhythmia, which is frequent in older adults, by explaining the physiopathology of one of its side effects: hyperthyroidism. TSH should be measured in patients with recurrent atrial fibrillation under treatment in order to exclude an amiodarone-induced thyroid dysfunction. In the geriatric population, the control of ventricular rate is often preferred over a conversion in sinus rhythm because the anti-arrhythmic drugs used for conversion have serious side effects.