Iodine is an element (atomic number 53) that is required by humans for the synthesis of thyroid hormones (triiodothyronine/T3 and thyroxine/T4). Chronic iodine deficiency can lead to numerous health problems in children and adults, including thyroid gland dysfunction (including goiter) and various neurologic, gastrointestinal, and skin abnormalities. Iodine deficiency in pregnant or nursing mothers can lead to significant neurocognitive deficits in their infants. "Cretinism" or severe mental retardation is a rare outcome of severe iodine deficiency during early development. Growth stunting, apathy, impaired movement, or speech/hearing problems may occur. Many individuals living in developing countries may be at risk of iodine deficiency and its complications, and iodine deficiency is considered to be a preventable cause of mental retardation. Iodine deficiency is rare in industrialized countries such as the United States, due to the enrichment of table salt and cattle feed with iodine. But deficiency is common in developing countries where supplementation may be considered. Humans obtain iodine from their diets. The amount of iodine in food or water depends upon the amount of iodine in the local soil. Areas with mountainous (glacier) water or heavy rainfall tend to be low in iodine content, increasing the risk of iodine deficiency. This review does not discuss medical uses of radioactive iodine or iodine contrast agents used for imaging studies such as computerized tomography (CT scanning).