Micronutrients: Iodine

Fish filet served with vegetables on white plate
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In the early 1920s, a voluntary salt iodization program was launched to help Americans consume adequate amounts of iodine. An essential micronutrient not commonly found in food, iodine is needed for the synthesis of thyroid hormones and is critical for normal neurodevelopment in utero, yet approximately 60 percent of pregnant women worldwide fall short of meeting the World Health Organization requirements. The WHO also estimates that 31.5 percent of school-age children and nearly 2 billion people worldwide do not consume adequate amounts of iodine.

Roles in Health

Seventy-five percent of total body iodine is stored in the thyroid gland, allowing it to make hormones necessary for physiologic processes including growth, reproductive function, brain development, healing, energy metabolism, central nervous system and healthy thyroid function.

Iodine supplements are used to treat conditions including fibrocystic breast disease and inflammatory skin conditions and are used as an emergency treatment for radiation exposure.

Current Recommendations

The current daily Adequate Intake is 110 micrograms for children 0 to 6 months and 130 micrograms for ages 7 to 12 months. The Recommended Dietary Allowance is 90 micrograms for children ages 1 to 8, 120 micrograms for ages 9 to 13 and 150 micrograms for ages 14 and older. Pregnant and lactating women require up to 220 micrograms and 290 micrograms, respectively.

Tolerable Upper Intake Levels are 200 micrograms per day for children 1 to 3 years, 300 micrograms per day for 4 to 8 years, 600 micrograms per day for 9 to 13 years, 900 micrograms per day for 14 to 18 years and 1,100 micrograms per day for adults.

Sources of Iodine

Iodine naturally is present in seawater and in varying amounts in soil. Iodized table salt and iodine-rich foods are reliable sources, but values can vary widely. Most salt used in processed foods is not iodized. Iodine is not listed on the Nutrition Facts Panel (unless the food has been fortified with it), nor in the USDA Nutrient Database.

Iodine also is available as a supplement and often is included in multivitamins.

Food sourcesRating
3 ounces baked cod99mcgExcellent
1 cup plain low-fat yogurt75mcgExcellent
112 grams (approx. 14 teaspoon) iodized salt71mcgExcellent
1 cup reduced-fat milk56mcgExcellent
2 slices enriched white bread45mcgExcellent
3 ounces shrimp35mcgExcellent
1 cup boiled enriched macaroni27mcgGood
1 large egg24mcgGood
3 ounces tuna, canned in oil, drained17mcgGood

Signs of Deficiency

The WHO estimates that more than 30 percent of the world’s population has inadequate iodine intake, as measured by urinary iodine below 100 micrograms per liter.

When a person is deficient in iodine, the results can range from a subtle loss of intelligence quotient to cretinism (the extreme condition of severe mental and physical retardation). Recent meta-analyses suggest iodine deficiency is a risk factor for thyroid cancer. Some researchers suggest deficiency also may be linked to prostate, breast, endometrial and ovarian cancers.

Iodine deficiency disorders include mental retardation, hypothyroidism, goiter (thyroid enlargement) and other growth and development abnormalities. Low levels of thyroid hormone can lead to infertility in women and autoimmune disease of the thyroid, thereby increasing risk of thyroid cancer.

The most serious effect of iodine deficiency occurs in utero with damage to the fetus. Low birth weight and decreased child survival also may result. Deficiency in the mother during pregnancy and in the child during the first two years of life can have negative cognitive consequences resulting in impaired speech development, learning, reading and potential behavior disorders.

Even mild iodine deficiency during pregnancy can have long-term adverse impacts on fetal neurocognition. During childhood, iodine deficiency has been linked to reduced intellectual and motor performance. Treatment and prevention includes iodine supplements and iodized salt.

Populations at Risk

Pregnant and lactating women and infants are most susceptible to iodine deficiency, yet many people are unaware of the importance of iodine during pregnancy and early life. This can be especially harmful as fetal development can be affected before signs of deficiency become apparent.

Cruciferous vegetables are high in goitrogens that can cause iodine deficiency, but people whose diets contain adequate iodine can safely enjoy these vegetables in normal amounts.

Anyone, especially women of child-bearing age, following a paleo-type diet could be at risk of iodine deficiency. Using iodized salt can help fill the micronutrient gap. People who eat a vegan diet may be at increased risk of iodine deficiency because they do not consume seafood or dairy. Iodized salt and sea vegetables are important sources of iodine for this population.

Toxicity

Iodine supplements likely are safe when taken by mouth or applied to skin in recommended amounts. Excess intake can cause symptoms similar to iodine deficiency.

Chronic excess iodine intake from iodine-rich foods such as kelp, supplements or water that is high in iodine can cause toxicity. Prolonged use of iodine supplements without medical supervision is potentially unsafe. Nausea, diarrhea, runny nose, headache and a metallic taste are common side effects of iodine toxicity.

Bottom Line

More research is needed on mild iodine deficiency and cognition. Further, attention is needed to help people understand and appreciate the importance of consuming adequate amounts of iodine. Salt iodization is an important strategy to ensure safe and effective levels of iodine consumption around the world, yet it must be balanced with efforts to control and reduce sodium intake.

Kathleen Zelman
Kathleen Zelman, MPH, RDN, is the nutrition director of WebMD.