Studies have shown that subsequent children of uncontrolled hypothyroid mothers have lower intelligence and that about 80% of such children have IQs below 85 (only 5% of children of non-hypothyroid mothers have IQs below 85), which is proportional to the degree of hypothyroidism. Although hypothyroidism in early pregnant women has little effect on the fetus, fetal brain development is significantly affected in pregnant women whose hypothyroidism is not well controlled by the 12th week of pregnancy. The fetal brain is developing all the time during pregnancy. The brain volume is increasing rapidly after the fetal organs are differentiated from 3 to 8 weeks of pregnancy, and the fetal brain is fully developed by the 5th month of pregnancy. During this period, the mother’s thyroxine level is crucial for the development of the fetal brain. The dosage of oral thyroxine in hypothyroid pregnant women often needs to be increased, and the specific dosage adjustment needs to refer to the TSH measurement, which is done every 2 months throughout the pregnancy, and after delivery, the need for thyroxine tablets decreases again (to the pre-pregnancy dosage), which can be adjusted according to the TSH level 2 months after delivery.