Iodine deficiency is a lack of the trace element iodine, an essential nutrient in the diet. It may result in metabolic problems such as goiter, sometimes as an endemic goiter as well as congenital iodine deficiency syndrome due to untreated congenital hypothyroidism, which results in developmental delays and other health problems. Iodine deficiency is an important global health issue, especially for fertile and pregnant women. It is also a preventable cause of intellectual disability. Iodine is an essential dietary mineral for neurodevelopment among children. The thyroid hormones thyroxine and triiodothyronine contain iodine. In areas where there is little iodine in the diet, typically remote inland areas where no marine foods are eaten, iodine deficiency is common. It is also common in mountainous regions of the world where food is grown in iodine-poor soil. Prevention includes adding small amounts of iodine to table salt, a product known as iodized salt. Iodine compounds have also been added to other foodstuffs, such as flour, water and milk, in areas of deficiency. Seafood is also a well known source of iodine.In the U.S., the use of iodine has decreased over concerns of overdoses since mid-20th century, and the iodine antagonists bromine, perchlorate and fluoride have become more ubiquitous. In particular, around 1980 the practice of using potassium iodate as dough conditioner in bread and baked goods was gradually replaced by the use of other conditioning agents such as bromide.Iodine deficiency resulting in goiter occurs in 187 million people globally as of 2010 (2.7% of the population).
A study of U.S. military data collected during the First and Second World Wars found that the introduction of salt iodization in the U.S. in the 1920s resulted in an increase in IQ, by approximately one standard deviation, for the quarter of the U.S. population most deficient in iodine, explaining about "one decades worth of the upward trend in IQ" in the U.S. (i.e., the Flynn effect). The same study documented "a large increase in thyroid-related deaths following the countrywide adoption of iodized salt, which affected mostly older individuals in localities with high prevalence of iodine deficiency".In iodine-deficient or mildly iodine-deficient areas of Europe, iodine deficiency is frequent during pregnancy despite the widespread use of iodised salt, posing risks to the neurodevelopment of foetuses. In one study performed in a mildly iodine-deficient area, iodine deficiency was found to be present in more than half of breastfeeding women; in contrast, the majority of their newborns had iodine excess, mostly due to neonatal exposure to iodine-containing disinfectants. A 2014 meta-analysis found that iodine supplementation "improves some maternal thyroid indices and may benefit aspects of cognitive function in school-age children, even in marginally iodine-deficient areas".