Effects of amiodarone therapy on thyroid function.

Amiodarone is a benzofuran derivative approved for the treatment of cardiac arrhythmias. Traditionally classified as a class III antiarrhythmic agent, amiodarone possesses electrophysiologic properties of all four Vaughan-Williams classes. This drug, however, has high iodine content, and this feature plus the intrinsic effects on the body make amiodarone especially toxic to the thyroid gland. Treatment can result in a range of effects from mild derangements in thyroid function to overt hypothyroidism or thyrotoxicosis. The diagnosis and treatment of amiodarone-induced hypothyroidism is usually straightforward, whereas that of amiodarone-induced thyrotoxicosis and the ability to distinguish between the type 1 and type 2 forms of the disease are much more challenging. Dronedarone was approved in 2009 for the treatment of patients with atrial fibrillation. As amiodarone, dronedarone is a benzofuran derivative with similar electrophysiologic properties. In contrast to amiodarone, however, dronedarone is structurally devoid of iodine and has a notably shorter half-life. In studies reported before FDA approval, dronedarone proved to be associated with significantly fewer adverse effects than amiodarone, making it a more attractive choice for patients with atrial fibrillation or flutter, who are at risk of developing amiodarone-induced thyroid dysfunction.