TABLE 6.

Mechanisms and causes of iodine excess¹

• Wolff and Chaikoff effect (59): high iodide exposure in rats has been shown to result in a transient inhibition of thyroid hormone synthesis lasting ∼24 h

◦Mechanism: possibly via the generation of intrathyroidal iodolactones or iodolipids, which inhibit TPO activity

◦Continued administration of iodide results in normal thyroid hormone synthesis referred to as “escape from the acute Wolff-Chaikoff effect” and may be caused by an inhibition of NIS synthesis, a reduction in intrathyroidal iodine, and a decrease in the iodine-induced inhibitors of hormone synthesis (57)

• The Jöd-Basedow phenomenon or iodine-induced hyperthyroidism represents a failure of the acute Wolff-Chaikoff effect, may occur in individuals with a history of nodular goiters caused by iodine deficiency

• Failure to “escape” from the acute Wolff-Chaikoff effect may also result in iodine-induced hypothyroidism

• Risk factors for iodine-induced hypothyroidism include underlying thyroid autoimmunity such as Hashimoto thyroiditis or a history of partial thyroidectomy

• Causes of iodine excess and related consequences might include the following:

◦Exposure to amiodarone, an antiarrhythmic medication that is 37% iodine by weight, or after exposure to iodinated radiographic contrast agents (34)

◦Ingestion of foods or supplements with very high iodine content, such as kelp, or where there is high iodine content in drinking water

◦Transient increases in rates of hyperthyroidism have been reported in historically iodine-deficient regions with the initiation of salt iodization

¹NIS, sodium/iodide symporter; TPO, thyroperoxidase.