Thyroid hormones, and therefore iodine, are essential for normal development of the brain. If a fetus or newborn is not exposed to enough thyroid hormone, it may have permanent mental retardation, even if it survives. The most damaging consequences of iodine deficiency are on fetal and infant development. Maternal iodine deficiency causes miscarriages, other pregnancy complications, and infertility. Low birth weights and decreased child survival also result from iodine deficiency. Cretinism is a very severe degree of this brain damage; it includes permanent dense mental retardation, and varying degrees of additional developmental defects such as deafmutism, short stature, spasticity, and other neuromuscular abnormalities.
The most visible consequence of iodine deficiency is goiter. This word means "an enlarged thyroid." The process begins as an adaptation in which the thyroid is more active in its attempts to make enough thyroid hormone for the body's needs, despite the limited supply of raw material (iodine), much as a muscle gets bigger when it has to do more work. If this adaptation is successful and the iodine deficiency is not too severe, the person may escape with only an enlarged thyroid and no other apparent damage from the iodine deficiency. Older individuals with goiters may develop nodules (lumps) in their thyroids, and sometimes these can begin making too much thyroid hormone when suddenly exposed to iodine. This result occurs because these nodules are independent of usual controls; they make thyroid hormone at their own rate, and may over-produce it when given more iodine. Also, the nodular goiters in iodine deficiency have an increased rate of one type of thyroid cancer, called "follicular cancer." Goiters can sometimes enlarge enough to produce compression of other neck structures and may need surgical removal for that reason.
In addition to these effects on the individual, iodine deficiency has adverse consequences for the community. The mental retardation can cover a wide range, from mild blunting of intellect to cretinism, and a large part of the population may have some intellectual impairment. The mean IQ of the deficient community is decreased by about 13.5 IQ points, according to one review. Individuals in these communities have lower educability and lower economic productivity, and the output of the whole community suffers. Dramatic improvement typically occurs after appropriate addition of iodine.
