Common causes of hypothyroidism in pregnancy include chronic autoimmune thyroiditis (Hashimoto’s disease), thyroid surgery, radioactive iodine therapy, and iodine deficiency. Physiological changes during pregnancy include: 1) increased thyroid binding globulin (TBG) and increased serum total T3 and T4 levels; 2) chorionic gonadotropin (hCG) concentrations gradually increase in the first to third trimester, reaching a peak in the third trimester, which decreases serum TSH during pregnancy; 3) during the second half of pregnancy, a significant increase in placental type III deiodinase, which can lead to a decrease in fetal TSH; 4) a significant increase in placental type III deiodinase, which can lead to a decrease in fetal TSH during pregnancy; 5) an increase in fetal TSH during the second half of pregnancy. Deiodinase is significantly increased in the second half of pregnancy, which can lead to fetal low T3 and high rT3 characteristics; 4. Increased renal clearance of iodine, fetal iodine demand, and increased levels of thyroid hormones in pregnancy can increase maternal iodine requirements. Adverse effects on the fetusWhen hypothyroidism in pregnancy is not corrected in time, it often leads to preterm delivery, low birth weight and neonatal respiratory distress, increased fetal or perinatal mortality, and also affects the neurological development and intelligence of the newborn. Treatment of hypothyroidism in pregnancy Women diagnosed with hypothyroidism prior to pregnancy and treated with levothyroxine (L-T4) supplementation should have their thyroid function tested and L-T4 dosage adjusted once pregnancy is confirmed. The currently recommended optimal time to take L-T4 is early in the morning on an empty stomach. If it is not tolerated, it can be delayed until her nausea and vomiting symptoms have subsided. Supplements such as iron, calcium and vitamins should be taken at least 2 hours apart from L-T4 to prevent them from forming compounds that would be detrimental to the absorption of L-T4. Generally, after 2 to 4 weeks postpartum, the thyroid hormone level returns to the pre-pregnancy level, so it is still necessary to continue to monitor the thyroid function after delivery to adjust the dosage of L-T4 in a timely manner. In addition, because female patients with autoimmune thyroid disease are at risk of developing postpartum thyroiditis, these patients should be monitored for at least 6 months after delivery.