Iodine-induced thyroid dysfunction
- Martin I Surks, MD
Martin I Surks, MD
- Professor of Medicine and Pathology
- Albert Einstein College of Medicine
The central role of iodide in thyroid physiology has been known for many years. The four iodine atoms of l-thyroxine (T4) constitute 65 percent of its weight; the three iodine atoms of triiodothyronine (T3) constitute 59 percent of its weight. Iodine is a trace element in the crust of the earth and its distribution is quite variable. Many areas, particularly inland and mountainous regions, have minimal iodine while others, often coastal regions, have sufficient or even excessive iodine.
Both iodine deficiency and excess can cause thyroid dysfunction. This topic will review the mechanisms by which normal subjects adapt to excess iodine and the mechanisms of iodine-induced thyroid disease. Iodine deficiency disorders and the therapeutic use of iodine in patients with hyperthyroidism are discussed separately. (See "Iodine deficiency disorders" and "Iodine in the treatment of hyperthyroidism".)
SOURCES OF IODINE
Iodide is essential for thyroid hormone synthesis. (See "Thyroid hormone synthesis and physiology".)
Iodine can be obtained by consumption of foods that contain it or to which it is added. Dietary iodine is absorbed as iodide and rapidly distributed in the extracellular fluid, which also contains iodide released from the thyroid and by extrathyroidal deiodination of the iodothyronines. Iodide leaves this pool by transport into the thyroid and excretion into the urine. The recommended minimum daily intake of iodine is 150 mcg for nonpregnant adults, 220 mcg for pregnant women, 290 mcg for lactating women, and 90 to 120 mcg/day for children aged 1 to 13 years . The average intake in the United States is about 150 to 200 mcg/day . In the United States, iodized salt contains 76 mcg of iodine/g. In many countries, however, it contains less, and in some countries iodized salt is not available. As a result, iodine deficiency is the most common cause of goiter, hypothyroidism, and mental deficiency worldwide. (See "Iodine deficiency disorders".)
Sources of excess iodine include over-the-counter and prescription medications that may be ingested or applied to the skin or vaginal mucosa, radiographic contrast agents, and dietary supplements (kelp, seaweed) (table 1). In the context of a person's usual dietary iodine intake, the amount of iodine in many of these substances is very large. As an example, a patient undergoing vascular imaging may receive several thousand mg of organic iodide. Those substances that contain organic iodide are partially deiodinated to form inorganic iodide, the form that has thyroidal actions. Some of these substances that are deiodinated to form inorganic iodine, such as amiodarone, are stored in fat and may provide excess iodide for months after the last dose is administered.
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- SOURCES OF IODINE
- NORMAL ADAPTATION TO IODIDE INTAKE
- Follicular cell autoregulation
- Iodide effects on thyroid hormone secretion
- IODINE-INDUCED THYROID DISEASE
- Iodine-induced hyperthyroidism
- - Diagnosis
- - Evaluation for underlying thyroid disease
- - Management
- - Prevention in older adults
- Iodide-induced hypothyroidism
- - Diagnosis and management
- SUMMARY AND RECOMMENDATIONS