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.
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.