Abstract: Iodine is an essential element for the thyroid hormone （ＴＨ） synthesis. It is well known that TH is involved in the regulation of genes related to fetus brain and neural development during pregnancy. The roles of TH in brain and neural development will even last to 3 years after birth. Fetal TH in the first and second trimesters originally comes from the free thyroxine （ＦＴ４） of pregnant women. Fetus in the last trimesters can produce TH which depends on the iodine supplements from pregnant women. The studies from the iodine-deficient areas showed that the iodine deficiency resulted in the brain impairment, and that the severity of brain impairment was related to the severity of iodine deficiency, such as endemic cretinism, subclinical endemic cretinism and intelligence impairments. The risks of TH deficiency and fetus brain impairment were increased even in those pregnant women with the latent iodine deficiency. The iodine replacement therapy in the iodine-deficient areas（iodized salt or iodized oil, etc.） can effectively prevent from offspring intelligence impairments. Excessive iodine intake usually happened in the areas with excessive iodine in drinking water. Studies of human being and animal suggested that the fetus developing brain can tolerate the excessive iodine in a certain extent. However, pregnant women should avoid the exposure to excessive iodine environment for long term. The test of 24 hours urinary iodine is available method for the evaluation of individual iodine nutrition. The iodized salt is a safe, effective and convenient supplement of iodine nutrition for those pregnant women with iodine deficiency. The adviced iodine intake is 200-500 μg/d for pregnant women.

Key words: Pregnant women, Fetus, Iodine, Thyroxine, Thyroid hormones, Brain, Intelligence