The diagnostic criteria for clinical hypothyroidism in pregnancy are: TSH > the upper line of the gestational reference value and FT4 < the lower line of the gestational reference value. The main cause of clinical hypothyroidism is autoimmune thyroiditis, and other causes include thyroid surgery. The prevalence of hypothyroidism in pregnancy in China is 1%. Several foreign studies have shown that hypothyroidism in pregnancy increases the risk of adverse pregnancy outcomes and may also have adverse effects on fetal neurointellectual development. Adverse pregnancy outcomes include preterm delivery, miscarriage, low birth weight, stillbirth, and hypertension during pregnancy. Foreign studies have shown that the risk of miscarriage increases by 60% in clinical hypothyroidism during pregnancy; other studies have shown that the risk of gestational hypertension increases by 22% and the risk of stillbirth also increases. Therefore, clinical hypothyroidism diagnosed during pregnancy should be taken seriously and treated actively. Treatment: L-T4 (Eugenol) is the preferred treatment for clinical hypothyroidism in pregnancy. Once the diagnosis of clinical hypothyroidism is confirmed, treatment should be started immediately to achieve the treatment goal as soon as possible. Women who already have clinical hypothyroidism planning pregnancy need to control serum TSH to <2.5mIU/L level before planning pregnancy. Pregnant women with clinical hypothyroidism should have their thyroid function tested every 4 weeks during the first half of pregnancy (1-20 weeks) and should have their thyroid function indicators tested at 26-32 weeks.