Adverse effects of iodine excess on the thyroid

Iodine excess can be harmful in patients with established thyroid disorders. Iodine exposure commonly results from:

• radiographic contrast agents
• iodine-containing topical preparations or drugs (eg amiodarone)
• complementary medicines (eg kelp tablets), which are often taken in the mistaken belief that iodine is routinely helpful for established thyroid disorders.

Topical iodine preparations can cause transient hypothyroidism in infants.

Iodine excess can precipitate drug-resistant hyperthyroidism in older patients with autonomous nodular thyroid disease (multinodular goitre or toxic adenoma). Iodine excess can also impair thyroid hormone formation in autoimmune hypothyroidism—correction of iodine excess has been reported to improve thyroid function in these patients.