All degrees of iodine deficiency (mild: iodine intake of 50-99 mug/day, moderate: 20-49 mug/day, and severe: <20 <mu>g/day) affect thyroid function of the mother and the neonate as well as the mental development of the child. The damage increases with the degree of the deficiency, with overt endemic cretinism as the severest consequence. Maternal hypothyroxinaemia during early pregnancy is a key factor in the development of the neurological damage in the cretin. Selenium deficiency combined with iodine deficiency partly prevents the neurological damage but precipitates severe hypothyroidism in cretins.

Iodine deficiency results in a global loss of 10-15 IQ points at a population level and constitutes the world's greatest single cause of preventable brain damage and mental retardation.