Iodine deficiency causes goitre, hypothyroidism, retarded physical development and impaired mental function, increased rate of spontaneous abortion and stillbirth, neurological cretinism, including deaf mutism and myxoedematous cretinism, dwarfism and severe mental retardation. It is the most common cause of preventable mental defect in the world today.
Iodine, the heaviest of the halogens, is required in proper quantities as an essential precursor to thyroid hormone synthesis. The thyroid gland converts the amino acid tyrosine into thyroglobulin, and attaches one to four iodine atoms to create T4 (thyroxine), the inactive storage hormone, T3 (triidothyronine), or metabolically active thyroid hormone, and two thyroid hormones whose clinical significance is less known, T2 and T1.
The major role of iodine in nutrition arises from the importance of these hormones to the growth and development of humans and animals. The daily requirement of iodine in adults is 1-2 micrograms per kilogram body weight. A daily iodine intake between 50 and 1000 micrograms is considered safe. The 1989 Recommended Daily Allowance (RDA) is in the range of 40-120 micrograms for children up to the age of 10 years and 150 micrograms for older children and adults. An additional 25 and 50 micrograms are recommended during pregnancy and lactation respectively.
Iodine deficiency causes depletion of thyroid iodine stores and reduced production of throxidine (T4), essential for normal brain development. A decrease in the blood concentration of T4 triggers the secretion of increased amounts of pituitary thyroid-stimulating hormone, which increases thyroid activity and results in hyperplasia (overactivity and depletion) of the thyroid gland. Conspicuous neurological features in significant to severe cases, described as cretinism (if a foetal or early childhood deficiency) or goitre if a later childhood or adult deficiency.
WHO estimated in 1990 that there were one billion people at risk of iodine deficiency disorders in developing countries. 190 million suffered from goitre and 20 million from mental defects due to iodine deficiency. Of these, more than 3 million were overt cretins. In 1997, 60% of the world's edible salt was iodized to combat the deficiency.
A 1995 survey of infants in China revealed that 35% to 65% had iodine deficiencies. A third of the global 1.6 billion iodine deficiency cases are in China. In 1997 in Georgia there was a widespread iodine deficiency and the 50,000 babies born there will be intellectually duller than normal by 10 IQ points.