Amiodarone and thyroid dysfunction.

Amiodarone is a potent and widely used antiarrhythmic drug that bears a structural resemblance to thyroid hormones. The high iodine content of the drug determines that amiodarone induces changes in circulating concentrations of thyroid hormones, largely through inhibition of conversion of thyroxine (T(4)) to tri-iodothyronine (T(3)). Amiodarone treatment typically results in a rise in serum T(4), often to above the normal range, associated with a fall in circulating T(3). These biochemical changes are found in subjects who remain clinically euthyroid. In addition to changes in circulating thyroid hormones found in euthyroid subjects, overt thyrotoxicosis and hypothyroidism may complicate amiodarone treatment. Amiodarone-induced thyrotoxicosis is more common in areas of iodine deficiency, whereas hypothyroidism is more common in iodine-rich parts of the world.