Iodine

Iodine (I) is a non-metallic element that the body needs in very small (trace) amounts to support thyroid function and overall health. Iodine is not produced by the body and can only be acquired through the diet. Iodine deficiency is a serious public health problem in some parts of the world where sources of iodine are lacking.

Iodine

SOURCE: U.S. Office of Dietary Supplements. “Iodine: Fact Sheet for Health Professionals.” National Institutes of Health. U.S. Department of Health and Human Services. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/#h4 (accessed April 14, 2018).

The thyroid gland is a tiny, butterfly-shaped organ located in the front of the neck just below the Adam's apple. It is part of a complex, tightly controlled feedback cycle that regulates the body's base (basal) metabolic rate, including how fast the body burns calories; the body temperature; and the rate of growth. It also helps to remove toxins from the body and helps in the utilization of minerals such as calcium and silicon. An iodine deficiency may have serious effects such as coarse skin, depression, intellectual disability, abnormal weight gain, decreased fertility, and increased risk of stillbirth in pregnant women.

The two main thyroid hormones are triiodothyronine (T3) and thyroxine (T4), each produced by the thyroid gland under stimulation by thyroid-stimulating hormone (TSH) made by the pituitary gland. Synthesis of one molecule of T3 requires three molecules of iodine, and synthesis of T4 requires four molecules of iodine. On average, the body needs between 100–200 mcg of iodine daily. When sufficient iodine is supplied by the diet, the body contains between 20 mg and 30 mg of iodine, 60% of which is stored in the thyroid. The remainder is found in trace amounts in the blood, muscles, and ovaries. Thyroid hormones are broken down in the liver and some of the iodine is recycled. The rest is excreted in urine.

Iodine is found in soil and in the ocean and coastal waters. Soil near coastal waters contains more iodine than inland areas. The amount of iodine varies widely by location. In mountainous regions where heavy rain and snow cause erosion, or in low-lying regions where regular flooding occurs, the soil is especially deficient in iodine. The mountains of the Himalayas, Andes, and Alps are all iodine-poor, as is the Ganges River Valley. The International Council for the Control of Iodine Deficiency Disorders (ICCIDD) estimates that 38% of the world's population, or about 2.2 billion people, live in areas where they are unlikely to get enough iodine without supplementation.

Iodine deficiency affects about two million people worldwide. Iodine deficiency disorders (IDDs) can create serious public health problems. In the early 1900s, iodine deficiency was common in interior regions of the United States and Canada, as well as many other noncoastal regions of the world. In the 1920s, the United States began a voluntary program of adding iodine (in the form of potassium iodide) to salt. Salt was chosen because all races, cultures, and economic classes use it, its consumption is not seasonal, and it is inexpensive. Adding 77 mcg of iodine per gram of salt costs about $0.04 per year per person in the United States. About 50% of table salt sold in the United States contains iodine. It is labeled “iodized salt.” All table salt sold in Canada is iodized. In most other countries, iodine is added at lower concentrations, ranging from 10–40 mcg/gram. Iodized salt programs must be monitored carefully so that an adequate iodine intake is maintained while also avoiding consumption of excess iodine.

The IOM has not set UL levels for iodine in children under one year old because of incomplete scientific information. RDAs for iodine are measured in micrograms (mcg). The following are the daily RDAs and AIs established for iodine to maintain thyroid function and health (they are the same as those recommended by the World Health Organization [WHO]):

Iodine must be acquired from the diet. Marine plants and animals, such as cod, haddock, herring, salmon, sardines, shellfish, and kelp or other seaweeds (e.g., dulse, wakame, arame, hijiki), are especially good sources of iodine because they concentrate the iodine found in seawater. Freshwater fish are not as good a source. Plant foods contain varying amounts of iodine, depending on the soil in which they are grown. Plant sources of iodine include Swiss chard, spinach, turnip greens, garlic, soybeans, and sesame seeds.

In industrialized countries, feed for cattle, chickens, and other domestic animals is often fortified with iodine. Some of this iodine finds its way into animal products that humans eat, including milk, eggs, and meat. In developing countries where feed is not enriched or cattle are raised on grass, these animal products do not serve as a source of iodine.

Commercially processed foods are often made with iodized salt. The iodine content of salt changes very little during processing. Sometimes an iodine-containing stabilizer is added to commercial bread dough, which increases the iodine content of bread. Commercially processed foods are the main source of iodine for most people in developed countries. Iodine is also a constituent in multivitamin tablets.

Iodine can be absorbed through the skin from iodine-based disinfectant solutions. Iodine in the air from automobile exhaust can be absorbed through the lungs, although this is not a desirable way to obtain iodine. Neither of these iodine sources provides significant amounts of iodine.

The following list gives the approximate iodine content of some common foods:

Because of iodine supplementation, iodine deficiency is not a serious health problem in most industrialized countries, but it remains a health risk in many developing countries. Internationally, about 2.2 billion people are at risk for IDDs. Women who do not get enough dietary iodine have higher rates of infertility, miscarriages, pregnancy complications, and low birthweight babies than women who have adequate iodine intake. The most damaging effects of iodine deficiency, however, occur in the developing fetus.

Iodine deficiency is the leading cause of abnormal brain development and preventable intellectual disability worldwide. Children born to iodine-deficient mothers have a condition called cretinism. Cretinism involves severe and permanent brain damage. These children have intellectual and developmental disorders such as deafness, mutism, and inability to control muscle movements.

The most visible sign of iodine deficiency in children, adolescents, and adults is the development of a goiter. A goiter is a small lump or larger growth near the throat that signals an enlarged thyroid. When not enough iodine is available, the thyroid grows larger in a futile attempt to make more thyroid hormone. In adults, hard lumps may form inside the goiter. When iodine deficiency is pronounced enough for a goiter to develop, memory and language skills decline, and in children IQ may be affected. Although these effects can often be reversed in children by increasing iodine intake, adults may not respond favorably. In adults with goiter, increasing iodine intake may send the thyroid into overdrive, causing it to produce too much thyroid hormone, a serious condition called hyperthyroidism. The thyroid gland may also produce too little hormone. Thyroid hormones are measured in a urine sample to determine if an individual is iodine deficient, and blood tests are performed to detect other thyroid function problems.

Pregnant and breastfeeding women must be especially careful to get enough iodine, because the most significant effects of iodine deficiency are found in fetuses and newborns. Vegans, who do not eat animal products and depend on soy for much of their protein, are at higher risk for iodine deficiency than the general population even though soybeans do contain iodine.

Amiodarone (Cordarone), a drug used to prevent irregular heart rhythms, contains enough iodine that it may affect thyroid function.

Some foods contain substances called goitrogens that interfere with the body's ability to absorb or use iodine, including broccoli, cabbage, cauliflower, and Brussels sprouts. Other foods that contain goitrogens are canola oil, soybeans, turnips, peanuts, and cassava. Large quantities of these foods would have to be consumed to cause iodine deficiency.

Selenium deficiency amplifies the effects of iodine deficiency. Vitamin A deficiency also may amplify iodine deficiency.

Complications of iodine deficiency in women include infertility, miscarriages, pregnancy complications, and low birthweight babies. In children, the problems include decrease in memory and language skills, intellectual and developmental disorders, and cretinism. Developing a goiter is a complication of iodine deficiency.

Iodine excess is only rarely caused by diet, although hyperthyroidism may result from other causes.

In developed countries, parents should have few concerns about their healthy children getting enough iodine if iodized table salt is consumed.

See also Minerals .