What is iodine-induced hyperthyroidism?

Iodine-induced hyperthyroidism is a condition that may develop when a susceptible patient is exposed to an excessive amount of iodine. In the normal state, the uptake and trapping of iodine by follicular cells in the thyroid gland is closely regulated. Safeguard mechanisms are in place that help to maintain steady levels of thyroid hormone production in the face of changing levels of iodine exposure. These safeguard mechanisms protect most individuals from exposure to excess amounts of iodine, inhibiting the production and release of excess amounts of thyroid hormone. Excess amounts of iodine may be present in certain types of foods that contain seaweed or kelp. Some formulations of cough syrups promoted as expectorants may contain iodine. Radiographic studies that use intravenous contrast agents to generate detailed images may be associated with exposure to excessive amounts of iodine. These may include intravenous pyelogram studies, contrast CT scans, cardiac catheterization procedures, and interventional radiology procedures.

For reasons that are somewhat unclear, the safeguard mechanisms that protect the thyroid gland may break down in some patients who are exposed to excess amounts of iodine. This may lead to production and secretion of excess amounts of thyroid hormone. Iodine-induced hyperthyroidism is more likely to develop in patients who live in areas of the world where there may not be enough iodine in the diet. It is also more likely to develop in patients who have preexisting autonomously functioning thyroid nodules or mild Graves' disease. In rare cases, iodine-induced hyperthyroidism may develop in patients who have completely normal thyroid glands.

A specific form of iodine-induced hyperthyroidism related to the use of a medication is called amiodarone-induced hyperthyroidism. Amiodarone is a cardiac medication that is widely used as an antiarrhythmic agent. It acts to suppress an abnormal cardiac rhythm called atrial fibrillation that develops when there is rapid disorganized contraction of the upper chambers of the heart. Each amiodarone tablet contains a significant amount of iodine. A certain percentage of all patients who are treated with amiodarone will develop thyroid conditions directly related to exposure to excess amounts of iodine. While patients treated with amiodarone are more likely to develop hypothyroidism characterized by underactivity of the thyroid gland, a few may actually develop hyperthyroidism. There appear to be two different types of hyperthyroidism that may develop in the setting of treatment with amiodarone. The first type is similar to other forms of iodine-induced hyperthyroidism. It is characterized by increased production and secretion of thyroid hormone. This type of amiodarone-induced hyperthyroidism may respond to treatment with antithyroid drugs. The second type, which is more accurately defined as amiodarone-induced thyrotoxicosis, appears to be a form of thyroiditis characterized by inflammation of the thyroid gland. This type of amiodarone-induced hyperthyroidism is more likely to respond to anti-inflammatory treatment with glucocorticoids.