Iodine is a trace mineral and an essential nutrient found naturally in the body.

Iodine is needed for the normal metabolism of cells. Metabolism is the process of converting food into energy. Humans need iodine for normal thyroid function, and for the production of thyroid hormones.

Iodized salt -- table salt with iodine added -- is the main food source of iodine. Seafood is naturally rich in iodine. Cod, sea bass, haddock, and perch are good sources. Kelp is the most common vegetable seafood that is a rich source of iodine. Dairy products also contain iodine. Other good sources are plants grown in iodine-rich soil.

Lack of enough iodine (deficiency) may occur in places that have iodine-poor soil. Many months of iodine deficiency in a person's diet may cause goiter or hypothyroidism. Without enough iodine, the thyroid cells and the thyroid gland become enlarged.

Deficiency happens more often in women than in men, and is more common in pregnant women and older children. Getting enough iodine in the diet may prevent a form of physical and mental retardation called cretinism. Cretinism is very rare in the U.S. because iodine deficiency is generally not a problem.

Iodine poisoning is rare in the U.S. Very high intake of iodine can reduce the function of the thyroid gland.