The diagnostic criteria for subclinical hypothyroidism in pregnancy are: serum TSH > the upper limit of the pregnancy-specific reference value (97.5 th) and serum FT4 within the reference range (2.5 th ~97.5 th). The reference range of pregnancy-specific serum thyroid indicators (referred to as gestational reference values) is divided into two categories, one is the gestational reference value established by the hospital or region, and the other is the reference value recommended by the guidelines. 2011 American Thyroid Association (ATA) guidelines first proposed the reference values for TSH specific to the three stages of pregnancy, i.e. T1 (0-13+6 weeks of gestation) stage, i.e. 0.1~2.5 in early pregnancy mIU/L; T2 (14-27+6 weeks of gestation), i.e. 0.2~3.0 mIU/L in mid pregnancy; T3 (28-41+6 weeks of gestation), i.e. 0.3~3.0 mIU/L in late pregnancy. Should subclinical hypothyroidism in pregnancy be treated? Subclinical hypothyroidism in pregnancy with positive TPOAb should be treated with L-T4. Subclinical hypothyroidism with negative TPOAb can be left untreated. The starting dose of L-T4 can be chosen according to the degree of TSH elevation. The starting dose of L-T4 was 100 μg/day. Adjust the dose of L-T4 according to the therapeutic target of TSH. What are the risks of subclinical hypothyroidism in pregnancy to the mother and child? Subclinical hypothyroidism in women during pregnancy increases the risk of adverse pregnancy outcomes and impairment of neurointellectual development in the offspring. However, due to insufficient evidence from evidence-based medicine, guidelines neither oppose nor recommend L-T4 treatment for pregnant women with TPOAb-negative subclinical hypothyroidism. Treatment, treatment goals and monitoring frequency for subclinical hypothyroidism in pregnancy are the same as for clinical hypothyroidism, and the therapeutic dose of L-T4 may be less than that of clinical hypothyroidism. Different doses of L-T4 may be given depending on the degree of TSH elevation.