Amiodarone, a drug used in the treatment of resistant arrhythmias, contains a large amount of iodine and has multiple effects on thyroid function at the pituitary-thyroid-tissue levels. While most patients receiving amiodarone remain clinically euthyroid, there are clear alterations in thyroid function tests, with slightly elevated FT4 and TSH, elevated rT3 and a low FT3 level. Amiodarone may cause hyperthyroidism (more commonly in areas of iodine deficiency) or hypothyroidism (more frequently in iodine-replete regions). In patients with amiodarone-induced hyperthyroidism, the FT3 level is characteristically raised, and levels of TSH are low or low-normal. The thyroid shows unusual histological features, with focal destructive changes against a background of inactive follicles. Amiodarone- induced thyrotoxicosis may be resistant to large doses of conventional antithyroid drugs and may require additional corticosteroid therapy.
