Before pregnancy: early screening and treatment Hypothyroidism can lead to infertility, and even after pregnancy, it is prone to hyperemesis, placental abruption, intrauterine distress, miscarriage, premature birth, low birth weight babies and stillbirth. More seriously, hypothyroidism during pregnancy (especially early pregnancy) can affect the brain and bone development of the fetus, resulting in the birth of a child with mental retardation and short stature, which is often referred to as “cretinism”. Because of the insidious onset of hypothyroidism and the lack of characteristic early symptoms (such as weakness, poor appetite, drowsiness, etc.), it is easy to be mistaken for a normal pregnancy reaction and be missed. If a patient does not know she has thyroid disease before pregnancy, the impact on the mother and the fetus is very great. Screening for thyroid disease in women who are planning to become pregnant or are already pregnant is essential from the point of view of the health of the mother and child. High-risk groups There is no consensus on the need to screen all pregnant women for thyroid function, but the consensus is to screen for hypothyroidism in high-risk groups before pregnancy. High-risk groups for hypothyroidism include: 1) those with a personal and family history of thyroid disease; 2) those with a history of goiter, thyroidectomy and 131I treatment; 3) those with previous findings of elevated TSH and positive autoantibodies to the thyroid gland; 4) those with a personal and family history of other autoimmune diseases. The timing of screening can be chosen before 8 weeks of gestation, preferably when pregnancy is planned. Screening indicators include serum TSH, FT4 and TPOAb, which is an independent risk factor for miscarriage. Women with clinically normal nail function and positive TPOAb only are at high risk of developing clinical hypothyroidism after pregnancy, which is why it is important to actively intervene even in patients with subclinical hypothyroidism who are positive for TPOAb. Pre-pregnancy preparation Before planning pregnancy, women with hypothyroidism should always have their nail function checked. If they have hypothyroidism, they should use temporary contraception and undergo L-T4 replacement therapy to bring their serum TSH to the pregnancy-specific normal range (TSH 0.3-2.5 mU/L) and FT4 to the upper 1/3 level of the normal range for non-pregnant women before allowing pregnancy. If a pregnant woman is found to be hypothyroid during pregnancy, she may choose to continue her pregnancy, but she should immediately start L-T4 replacement therapy to bring serum TSH up to standard as soon as possible (preferably within 8 weeks of gestation) to ensure adequate supply of thyroid hormones for the first period of rapid fetal brain development (i.e., within the 4th to 6th months of gestation).