Pregnancy and hypothyroidismClinical hypothyroidism is associated with reduced fertility in patients. Maternal hypothyroidism during pregnancy has been associated with gestational hypertension, placental abruption, spontaneous abortion, fetal distress, preterm labor, and the development of low birth weight infants. A 40-year retrospective survey showed that the prevalence of gestational hypertension was 3.8% and 11.6% in normal controls and clinical hypothyroidism, respectively; spontaneous abortion was 3.3% and 8.0%; preterm labor was 3.4% and 9.3%; perinatal fetal mortality was 0.9% and 8.1%; and low-birth-weight infants were 6.8% and 22%, respectively. There is not enough clinical information on the complications of pregnancy in subclinical hypothyroidism. In recent years, the effect of maternal subclinical hypothyroidism in early pregnancy on the first stage of fetal brain development has received much attention. Before fetal thyroid function is fully established (i.e., before 20 weeks of gestation), all thyroid hormones required for fetal brain development originate from the mother, and maternal thyroid hormone deficiency can lead to impaired neurointellectual development in the offspring. American scholars Haddow et al. found for the first time: 17 weeks of pregnancy with subclinical hypothyroidism in mothers, not given levothyroxine treatment group of mothers of 7-9 years old offspring of the intelligence quotient (IQ) than the offspring of mothers in the normal control group decreased by 7 points. In contrast, the IQ of the offspring of the group given L-T4 treatment did not differ from that of the offspring of the normal control group. Reference ranges for TSH and thyroid hormones during pregnancy differ from those of the general population due to a variety of factors. There is no pregnancy-specific TSH reference range. It is generally believed that the TSH reference range in early pregnancy should be 30-50% lower than that of the non-pregnant population. Currently, some international scholars propose 2.5 mU/L as the upper limit of the normal range of TSH in early pregnancy, beyond which subclinical hypothyroidism in pregnancy can be diagnosed. Due to the large fluctuation of FT4, it is internationally recommended to apply TT4 to evaluate the thyroid function of pregnant women. The concentration of TT4 increases during pregnancy and is approximately 1.5 times higher than in non-pregnancy. If TSH is normal during pregnancy (0.3-2.5 mU/L) and only TT4 is below 100 nmol/L (7.8 μg/dL), a diagnosis of hypo-T4emia can be made. The initial brain development of the fetus is directly dependent on the level of T4 in the maternal circulation, but not on the level of T3. Treatment: Hypothyroidism that has been diagnosed prior to pregnancy requires adjustment of the L-T4 dose to bring serum TSH within the normal range before pregnancy is considered. The L-T4 replacement dose is usually increased by 30%-50% during pregnancy compared to the nonpregnant state. If there is no history of hypothyroidism and hypothyroidism is diagnosed during pregnancy, L-T4 therapy should be administered immediately, with the goal of bringing serum TSH to the range of normal values specific for pregnancy as soon as possible. Some foreign scholars suggest that this range should be 0.3-2.5 mU/L. The earlier the standard is reached, the better (preferably within 8 weeks of pregnancy). TSH, FT4 and TT4 should be measured every 2-4 weeks, and the dose of L-T4 should be adjusted according to the results of the monitoring.After TSH reaches the standard, TSH, FT4 and TT4 should be monitored every 6-8 weeks.Prospective studies on interventions in pregnant women with subclinical hypothyroidism, hypo-T4emia and TPOAb positivity are underway in several countries, and there is no unanimous agreement on the treatment at this point in time. The three societies mentioned above (ATA, AACE, TES) advocate routine TSH screening of pregnant women for timely detection and treatment of clinical hypothyroidism and subclinical hypothyroidism. The prevalence of suboptimal hypothyroidism in women of childbearing age is around 5%. Some scholars have advocated pre-pregnancy screening for those at high risk of developing hypothyroidism. High-risk groups for hypothyroidism include: those with personal and family history of thyroid disease; those with history of goiter and thyroid surgery and 131I treatment; those with personal and family history of autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes mellitus, and those who have been found to have elevated serum TSH or positive serum thyroid autoantibodies, and so on. Education on the effects of hypothyroidism on pregnancy and fetal brain development should be enhanced for women of childbearing age who already have hypothyroidism.