Iodine is a trace element found in some foods, fortified in others and also available in supplement form. Its most important role in the body is to help promote regular functioning of the thyroid, which is responsible for protein synthesis and ensuring proper metabolism. Iodine is found in soil, which affects the iodine content of the food that is grown in it. Soil with diminished iodine is common in many parts of the world, table salt is regularly fortified with it to reduce the prevalence of deficiency.

Iodine is found in a variety of foods, many of which have widely varied amounts due to production techniques. The foods with the largest amount of iodine include seaweed, cod, shrimp, prunes, lima beans and iodized salt. Iodized salt and enriched grains tend to be the major sources of dietary iodine in the typical American diet. Salt manufacturers in the U.S. have been fortifying their product with iodine since the 1920s.

Iodine deficiency used to be very common in mountainous regions of the United States as well as around the Great Lakes, as those areas tend to have soils with low iodine content. Deficiency is now very uncommon in developed countries but remains a problem in much of the developing world. The groups of people who are most likely to consume insufficient iodine include those who live in areas with low soil iodine content, people who do not have access to iodized salt, and pregnant women.

Iodine is essential to proper fetal and infant development, so it is important that pregnant and breast feeding women consume sufficient levels. Severe iodine deficiency during childhood has been shown to reduce cognitive function as measured by IQ by 13 points on average. Even mild iodine deficiency in children can cause cognitive problems. Health problems related to too much iodine intake mirror those caused by insufficient intake, as it causes the thyroid to quit functioning properly.