Iodine is an integral part of thyroid hormones. T4 (thyroxine) contains four iodine atoms, and T3 (triiododthyronine), the active form of thyroid hormone, contains three. Thyroid hormones regulate metabolism, growth, development, and reproductive function. In order for these hormones to be made in the body, iodine must be present. The thyroid gland concentrates iodine from the blood by means of a transporter molecule called the sodiumiodide symporter (NIS). Insufficiency of iodine can result in hypothyroidism, goiter, stunted growth, retarded psychomotor development, and irreversible mental retardation (cretinism). The importance of iodine for proper development of the fetus is suggested by the fact that NIS is activated in the lactating breast, which then concentrates iodine and secretes it into milk.

Most of the iodine in the American diet comes from dairy products, eggs, and iodized salt. Iodized salt contains 77 micrograms iodine/gram of salt. (A teaspoon of salt weighs just under 6 grams.) Dairy products and eggs from animals with iodine added to their feed are rich sources of iodine. Seafood is an excellent source of iodine. Seafood obtains iodine from seawater and seaweed that are rich in iodine. Seafood like haddock, cod, sea bass and perch, as well as sardines, shrimp, clams, lobsters, and oysters are rich sources of iodine. Seaweeds like kelp, wakame, nori, dulse, and hijike are rich sources of iodine as well.

Iodine supplements are available most frequently as potassium iodide or as a combination of elemental iodine and potassium iodide. Iodine is the free form of the mineral and iodide has been attached to another molecule, such as potassium; the body recognizes both forms. Seaweed, an excellent source of naturally-occurring iodine, is also available in capsule form.

The Recommended Dietary Allowance (RDA) for iodine is 9-20 mcg/day for children; 150 mcg/day for adults; 220 mcg/day for pregnant women; and 290 mcg/day for breastfeeding women. These levels are considered sufficient for prevention of goiter and cretinism and also for producing normal blood levels of thyroid hormones. Due to serious health risks, consult a doctor before supplementing with iodine if you are considering taking an amount above the RDA.

Excess iodine intake can disrupt normal thyroid function. Individuals with iodine deficiency, nodular goiter, or autoimmune thyroid disease may be sensitive to intake levels considered safe for the general population. Children with cystic fibrosis may also be more sensitive to the adverse effects of excess iodine. 

Hypothyroidism (too-low levels of thyroid hormones) can also be a result of excess iodine intake. High levels of iodine suppress thyroid hormone production. Certain individuals may require less iodine to suppress hormone formation than the average person. Those people thought to be at risk for iodine-induced hypothyroidism include those with underlying thyroid problems; fetuses (pregnant women should be cautious in taking more than the RDA) and newborn infants; those with goiter due to very high iodine intake, elderly people without clinical thyroid disease but who may have subclinical hypothyroidism (elevated thyroid stimulating hormone (TSH) but normal levels of T4); people with a family history of goiter or thyroiditis; and people taking lithium.