The critical period of human brain development is from the beginning of pregnancy to 3 years after birth. During the critical period of brain development, the development of the nervous system depends on the regulation of thyroid hormones. If pregnant women and infants living in iodine-deficient areas are threatened by iodine deficiency and cannot synthesize enough thyroid hormones, they will suffer from brain developmental damage. The intellectual development of children living in different iodine deficient areas can be harmed by iodine deficiency, only the degree of intellectual impairment varies. Iodine deficiency during pregnancy causes more mental impairment than later in life. The most severe mental impairment in children due to iodine deficiency is cretinism, which is called severe mental retardation combined with congenital iodine deficiency syndrome in the Classification of Diseases System-10 issued by the World Health Organization. In areas of severe iodine deficiency, children’s IQ levels lag behind by an average of about 10-15 percentage points, and the prevalence of children with mental retardation is 10-15%. Other mental disorders caused by iodine deficiency include mild mental retardation and decreased level of intelligence. The conditions for the diagnosis of mental retardation are age of onset less than 18 years, two IQ test scores below 70 IQ points, and low social adjustment assessment. Biological factors are the basis and prerequisite for normal intellectual development and determine the level of individual potential. Iodine deficiency disrupts the biological basis of normal intellectual development by reducing thyroid hormone synthesis. If we rely only on acquired education and intervention, we can only reduce the degree of mental impairment, but not recover the entire damage. Therefore, mental impairment due to iodine deficiency is irreversible. Among the factors that hinder normal intellectual development, iodine deficiency is the most clearly preventable factor. Universal salt iodization in iodine-deficient areas can successfully prevent mental impairment in children due to iodine deficiency. In iodine deficient areas, continuous appropriate iodine supplementation measures are very important, and any factor that leads to interruption of the measures may have serious consequences in the form of new onset of cretinism.