Iodine: Benefits, Dosage, and Safety

What iodine is

Iodine is a trace element naturally present in some foods, added to some types of salt, and available as a dietary supplement. It is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Because iodine rarely occurs as the free element but rather as a salt, it is often referred to as iodide. Iodide is quickly and almost completely absorbed in the stomach and duodenum, and the thyroid gland concentrates it for hormone synthesis while most of the remainder is excreted in the urine. An iodine-replete healthy adult holds about 15 to 20 mg of iodine, 70 to 80 percent of which is in the thyroid.

What it does

• Serves as an essential building block of the thyroid hormones thyroxine (T4) and triiodothyronine (T3)
• Supports thyroid hormones that regulate metabolic activity, protein synthesis, and enzymatic activity throughout the body
• Required for proper skeletal and central nervous system development in fetuses and infants
• May play a role in immune response
• Appears to have a beneficial effect on mammary dysplasia and fibrocystic breast disease, though the mechanism is not established

How much iodine per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Goiter (enlargement of the thyroid gland) is usually the earliest clinical sign of iodine deficiency. Intakes below about 100 mcg/day raise TSH; if intake falls below roughly 10 to 20 mcg/day, hypothyroidism can occur, frequently accompanied by goiter. In pregnancy, deficiency of this magnitude can cause major fetal neurodevelopmental deficits, growth retardation, miscarriage, and stillbirth. Chronic, severe deficiency in utero causes cretinism (intellectual disability, deaf mutism, motor spasticity, stunted growth, and delayed sexual maturation). In infants and children, less severe deficiency can cause neurodevelopmental deficits including somewhat lower IQ. Iodine deficiency is the most common cause of preventable intellectual disability in the world.

Who may need more

• Women who are planning a pregnancy, pregnant, or lactating (the American Thyroid Association and American Academy of Pediatrics recommend a supplement providing at least 150 mcg/day iodine as potassium iodide)
• People who do not use iodized salt
• Vegans and others who eat little or no seafood, dairy, or eggs
• People living in areas with iodine-deficient soils without access to iodized salt or foods from outside the area

Food sources

Seaweed (kelp, nori, kombu, wakame) is among the best sources but content varies enormously (16 to 2,984 mcg/g). Other good sources: cod (about 146 mcg per 3 oz), other fish and seafood, oysters, and eggs (about 31 mcg per large egg). Dairy provides substantial amounts (nonfat milk about 84 mcg/cup; Greek yogurt about 87 mcg per 3/4 cup). Iodized table salt provides about 78 mcg per 1/4 teaspoon (U.S. iodized salt contains about 45 mcg iodine/g). Bread made with iodate dough conditioners can be high (about 296 mcg per 2 slices), though only about 20 percent of U.S. breads use them. Most fruits and vegetables are poor sources.

Forms and absorption

In dietary supplements iodine is often present as potassium iodide or sodium iodide; supplements containing kelp (a seaweed) are also available. Potassium iodide is almost completely absorbed (about 96.4 percent in one small study). Many multivitamin/mineral supplements contain iodine, commonly at 150 mcg, but some, and not all, prenatal supplements contain it. Iodine-only supplements are available and many contain high doses, sometimes above the UL, so check the Supplement Facts label.

Safety, excess and interactions

Too much

High iodine intakes can cause some of the same symptoms as deficiency, including goiter, elevated TSH, and hypothyroidism, because excess iodine in susceptible people inhibits thyroid hormone synthesis. Iodine-induced hyperthyroidism (thyrotoxicosis) can also occur, including when iodine is given to treat deficiency. Excessive intakes have been shown to cause thyroiditis and thyroid papillary cancer. People with autoimmune thyroid disease or pre-existing iodine deficiency may react adversely to intakes considered safe for the general population. Acute iodine poisoning is rare and usually results from doses of many grams; symptoms include burning of the mouth, throat, and stomach, fever, abdominal pain, nausea, vomiting, diarrhea, weak pulse, and coma.

Interactions to know

• Antithyroid medications (for example methimazole, Tapazole), used to treat hyperthyroidism: taking high doses of iodine with them can have an additive effect and could cause hypothyroidism.
• Potassium-sparing diuretics (for example spironolactone, Aldactone; amiloride, Midamor): taking potassium iodide together can increase the risk of hyperkalemia (high blood potassium).
• ACE inhibitors (for example benazepril, Lotensin; lisinopril, Prinivil and Zestril; fosinopril, Monopril): taking potassium iodide together can increase the risk of hyperkalemia.

How status is measured

Iodine status is typically assessed with urinary iodine concentration, because more than 90 percent of dietary iodine is excreted in the urine. Median urinary iodine concentrations of 100 to 199 mcg/L in children and nonpregnant adults, 150 to 249 mcg/L in pregnant women, and greater than 100 mcg/L in lactating women indicate adequate intake. Spot urine samples characterize the status of populations but are not reliable for diagnosing individuals; multiple 24-hour or multiple spot measurements are more accurate for an individual. Thyroglobulin and TSH are related markers, but TSH is not a sensitive indicator of iodine deficiency.

Iodine is an essential nutrient with a Daily Value (DV) of 150 mcg for adults and children age 4 and older. It is sold as a dietary supplement (often potassium iodide or sodium iodide), which the FDA regulates as food, not as a drug, so products are not FDA-approved for safety or efficacy before marketing. FDA has approved potassium iodide as a thyroid-blocking drug for radiation emergencies, and has approved potassium iodide and cuprous iodide for salt iodization. Salt iodization in the United States is voluntary.

What the evidence supports

We grade the strength of evidence behind each common reason people take iodine, mirroring how the NIH describes it.