In recent years, the incidence of thyroid disorders has increased significantly and the age of onset is younger. What should a pregnant mother or mother-to-be do when facing pregnancy with thyroid disease in pregnancy or a history of previous thyroid disease? Only hypothyroidism is discussed in me. Definition of clinical (or subclinical) hypothyroidism in pregnancy and hypothyroxinemia in pregnancy Because of the widespread clinical use of ultrasensitive thyroid stimulating hormone (TSH) tests and the TSH-like effect of human chorionic gonadotropin (hCG) secreted by the placenta during pregnancy, the ATA has redefined normal TSH values in pregnancy: the upper limit of normal TSH in early pregnancy is 2.5 mIU/L, and in mid- and late-pregnancy is If a pregnant woman’s TSH exceeds the upper limit of the normal value for the corresponding gestational period, she should be considered to be hypothyroid. Clinical hypothyroidism should be considered when TSH is ≥10 mIU/L in pregnant women, regardless of whether free thyroxine (FT4) is normal or not. Subclinical hypothyroidism in pregnancy is defined as TSH in the range of 2.5 to 10 mIU/L with normal FT4. Hypothyroidism in pregnancy refers to a pregnant woman with normal TSH but FT4 below the 5th (very hypothyroidism) or 10th percentile values (hypothyroidism) of the reference range. Dangers of hypothyroidism in pregnancy Clinical hypothyroidism can increase the risk of pregnancy complications and may increase the risk of neurocognitive defects during fetal development. Regarding the effect of subclinical hypothyroidism in pregnant women on fetal neurocognitive development, the ATA considers it biologically plausible but the clinical evidence is not sufficient. There is also controversy regarding the negative impact of gestational hypothyroxinemia on the fetus. Treatment of hypothyroidism in pregnancy Oral levothyroxine (LT4) therapy is recommended, targeting TSH ranges of 0.1 to 2.5 mIU/L, 0.2 to 3.0 mIU/L and 0.3 to 3.0 mIU/L in early, mid and late pregnancy, respectively. Monitoring of hypothyroid pregnant women The body’s requirement for LT4 varies during pregnancy. 50% to 80% of hypothyroid pregnant women’s LT4 dosage during pregnancy must be The amount of LT4 must be increased by 20% to 50% to meet the needs of the body. From the 4th to 6th week of gestation, the demand for exogenous LT4 starts to increase until the 16th to 20th week of gestation; after the 20th week of gestation, there is no significant change in the demand for LT4 until delivery; after delivery, the demand for LT4 returns to the prenatal level. Therefore, pregnant women using LT4 should have their TSH and FT4 checked every 4 weeks until 16-20 weeks of gestation, and TSH should be measured once every 6 weeks after delivery. What TSH levels should be maintained in LT4-treated patients who are preparing for pregnancy? Because hCG secreted by the placenta after pregnancy stimulates the thyroid gland to produce more thyroid hormone, TSH is significantly lower in pregnant women. To avoid elevated TSH in early pregnancy, LT4-treated hypothyroid women should maintain a TSH <2.5 mIU/L until they are ready to become pregnant. Can LT4 be used in patients with normal thyroid function but positive thyroid antibodies who are ready to become pregnant? These patients have a low conception rate, a high rate of miscarriage after pregnancy, a low success rate of assisted reproduction, and a tendency to have elevated TSH after pregnancy. There are no studies on the effect of LT4 on women with purely positive thyroid antibodies in pregnancy. It is recommended to test thyroid function every 4-6 weeks after pregnancy and to treat with LT4 if TSH exceeds the upper limit of normal value in pregnancy.