iodine

Iodine is an essential trace mineral found naturally in some foods, added to iodized salt, and sold as a dietary supplement (usually as potassium iodide or sodium iodide, and also in kelp products and many multivitamins). According to the NIH Office of Dietary Supplements, the body needs iodine to make thyroid hormones, which regulate metabolism and are also required for normal bone and brain development during pregnancy and infancy. Most people in the United States get enough iodine from food and beverages, and iodine deficiency is uncommon in the US and Canada.

Iodine's role in making thyroid hormones is well established, and severe deficiency clearly harms fetal, infant, and childhood development. For other uses the NIH is more cautious. For mild deficiency and cognitive function, supplements appear to improve mental and physical development in iodine-deficient children, but the NIH states that "more study is needed to fully understand the effects of mild iodine deficiency and of iodine supplements on cognitive function." For fibrocystic breast disease, the NIH says very high doses "might reduce the pain and other symptoms... but more study is necessary to confirm this." There is no good evidence that extra iodine benefits people who already get enough, and the NIH notes that high doses can be unsafe.

Recommended daily amounts (from all sources) per the NIH are 90 mcg for children 1-8 years, 120 mcg for ages 9-13, 150 mcg for teens 14+ and adults, 220 mcg during pregnancy, and 290 mcg while breastfeeding. The American Thyroid Association and American Academy of Pediatrics advise women who are pregnant, planning pregnancy, or breastfeeding to take a daily supplement containing 150 mcg iodine as potassium iodide. The tolerable upper intake level for adults (food plus supplements) is 1,100 mcg/day, with lower limits for children. Talk to your doctor, pharmacist, or registered dietitian before starting iodine supplements, especially if you are pregnant, have a thyroid condition, or take other medications.

Both too little and too much iodine can cause problems. The NIH notes that high iodine intakes can cause some of the same effects as deficiency, including goiter (an enlarged thyroid gland), and can also cause thyroid gland inflammation and thyroid cancer. The daily upper limits apply to intake from all sources and do not apply to people taking iodine for medical reasons under a doctor's care. People with thyroid disorders, those who are pregnant or breastfeeding, and anyone considering high-dose iodine (for example for fibrocystic breast disease) should check with a healthcare provider first.

At recommended amounts iodine is generally well tolerated. Getting too much can lead to goiter, thyroid inflammation, and increased thyroid cancer risk, according to the NIH. The NIH also warns that a very large single dose on the order of several grams can be serious, producing symptoms such as a burning sensation in the mouth, throat, and stomach, along with fever, abdominal pain, nausea, vomiting, diarrhea, a weak pulse, and coma.

The NIH lists several drug interactions. Combining high doses of iodine with antithyroid medications such as methimazole (Tapazole), used for hyperthyroidism, may have an additive effect that leaves the body producing too little thyroid hormone. Taking potassium iodide alongside ACE inhibitors for high blood pressure (for example benazepril, lisinopril, or fosinopril) may push blood potassium to unsafe levels. The same risk applies when potassium iodide is combined with potassium-sparing diuretics such as spironolactone (Aldactone) or amiloride (Midamor). Tell your doctor and pharmacist about all the supplements and medicines you take.