Amiodarone and thyroid dysfunction

Amiodarone is a benzofuranic derivative, iodine-rich drug widely used for treatment of cardiac tachyarrhythmias. It often leads to transient changes in thyroid function tests, either due to the high iodine content of the drug or due to the direct toxic effect of the drug. The prevalence of amiodarone induced thyroid dysfunction depends on dietary iodine intake, age and sex of the patient and also presence of underlying thyroid disorder. Thyroid dysfunction occurs in about 15% of patients treated with amiodarone (Amiodarone induced hypothyroidism (AIH) or Amiodarone induced thyrotoxicosis (AIT)). AIH is commoner in iodine sufficient areas and in patients with a background of Hashimoto's thyroiditis and is related to failure to escape from the acute Wolff Chaikoff effect. It may develop as early as 2 weeks or as late as 3 years following initiation of amiodarone therapy. Treatment of AIH consists of levothyroxine replacement, while continuing amiodarone therapy. AIT is more complex and there are two varieties namely AIT type I and AIT type II. AIT type I is commoner in iodine deficient areas and typically occurs with underlying thyroid disease (due to Jod Basedow phenomenon). There is some benefit in stopping amiodarone in such a scenario and treating with thionamides with or without potassium perchlorate. AIT type II on the other hand is commoner in iodine sufficient areas and occurs due to the direct toxic effect of amiodarone on thyroid follicular cells. Withdrawal of amiodarone with or without glucorticoids is useful. Mixed forms are best treated with a combination of thionamides, potassium perchlorate and glucocorticoid. Thyroidectomy is contemplated in resistant cases. Radio-iodine is usually of little benefit due to low thyroidal radioiodine uptake. Thyroid function tests should be performed at baseline prior to initiation of amiodarone therapy. Clinicians should also note presence of any goiter and document any history of autoimmune disorders. Thereafter thyroid function tests need to be repeated at 3-6 month interval initially and thereafter annually for at least a year following withdrawal of therapy. This is of utmost importance to detect amiodarone induced thyroid problems. A significant proportion of patients on amiodarone are also on Warfarin and it is important to realize that amiodarone as well as thyroid dysfunction alters the pharmacokinetics of warfarin. Therefore adjustments of warfarin dosage have to be made accordingly. A newer drug, namely dronedarone is currently under review. As dronedarone does not contain iodine it is hoped that it will have the same benefits but not the ill effects on thyroid function.