The fetus is totally dependent in early pregnancy on maternal thyroxine for normal brain development. Adequate maternal dietary intake of iodine during pregnancy is essential for maternal thyroxine production and later for thyroid function in the fetus. If iodine insufficiency leads to inadequate production of thyroid hormones and hypothyroidism during pregnancy, then irreversible fetal brain damage can result. In the United States, the median urinary iodine (UI) was 168 microg/L in 2001-2002, well within the range of normal established by the World Health Organization (WHO), but whereas the UI of pregnant women (173 microg/L; 95% CI 75-229 microg/L) was within the range recommended by WHO (150-249 microg/L), the lower 95% CI was less than 150 microg/L. Therefore, until additional physiologic data are available to make a better judgment, the American Thyroid Association recommends that women receive 150 microg iodine supplements daily during pregnancy and lactation and that all prenatal vitamin/mineral preparations contain 150 microg of iodine.