Thyroid hormone and intellectual development: a clinician's view.

Cretinism, which is characterized by marked intellectual impairment and is associated with severe iodine deficiency, has been appreciated for many years. Advances in the knowledge of thyroid physiology and its changes in normal pregnancy have now led to the realization of the potential importance of minor changes in maternal thyroxine (T4) concentrations in relation to brain maturation and development. Mild and subclinical neuropsychomotor deficits have been observed in neonates both in mildly iodine-deficient areas and in iodine-sufficient areas where maternal T4 concentrations are in the low normal range. Recent data from Holland suggest that children born to mothers known to have circulating antithyroid peroxidase antibodies or from mothers with low normal free T4 concentrations measured at 12 weeks gestation have significant development impairment. There are important implications for possible screening for thyroid dysfunction during pregnancy arising from these studies.