Amiodarone and thyroid function

Amiodarone is a class III antiarrhythmic medication that contains 37% iodine by weight, structurally similar to thyroid hormones. The deiodination of amiodarone releases substantial amounts of iodine, which can disrupt thyroid function and lead to either hypothyroidism or thyrotoxicosis in approximately 20% of patients receiving the drug. This thyroid dysfunction is caused not only by the excess iodine but also by the direct cytotoxic effects of amiodarone and its metabolite, desethylamiodarone, on thyroid cells.

Hypothyroidism is more likely to occur in individuals with preexisting Hashimoto's thyroiditis, although it can also develop in patients without any morphological or serological thyroid abnormalities. Amiodarone-induced thyrotoxicosis can present in two forms:

1. Type 1: Iodine-induced thyrotoxicosis, occurring in individuals with underlying thyroid autonomy or latent Graves' disease.
2. Type 2: Destructive thyroiditis, where direct cytotoxicity leads to the release of preformed thyroid hormones from damaged thyroid cells.

Amiodarone's impact on thyroid function necessitates careful monitoring of thyroid function tests and appropriate management strategies for affected patients.