Iodine and Hyperthyroidism: A Double-Edged Sword.

Iodine is a trace element found in the Earth's crust that is necessary for the biosynthesis of thyroid hormones. Excessive iodine exposure can lead to hyperthyroidism due to the failure of normal homeostatic mechanisms. Iodine-induced hyperthyroidism occurs most frequently in historically iodine-deficient regions, where there is an increased prevalence of autonomously functioning thyroid nodules, but this can also occur in regions with optimal background iodine intakes. Potential sources of iodine excess include iodized salt, seaweed, iodine-containing supplements, drinking water, iodinated contrast media, and amiodarone. In addition to being a potential cause of hyperthyroidism, inorganic iodine may be used to treat Graves' hyperthyroidism and thyroid storm, although there is a risk that this may worsen hyperthyroidism in some patients. Inorganic iodine is also used as a preoperative treatment to reduce thyroid vascularity before thyroidectomy. Recognizing potential sources of iodine and patients who may be particularly vulnerable to the effects of iodine excess can help to identify and prevent hyperthyroidism. This article serves as an update to a 1995 review by Braverman and Roti that was published in the inaugural issue of Endocrine Practice.