Hypothyroidism (hypothyroidism) is a disease in which the body’s metabolism is reduced due to a decrease in the synthesis and secretion of thyroid hormones or their physiological effects. Depending on the cause, hypothyroidism can be divided into primary hypothyroidism, secondary hypothyroidism, hypothyroidism due to thyroiditis, and thyroid hormone resistance hypothyroidism. Patients with hypothyroidism have a low metabolism, which is characterized by fear of cold, weakness, dry skin, swelling of the face and eyelids, drowsiness, memory loss, bradycardia, decreased appetite, constipation, abdominal pain and bloating, and withered hair and hair loss. Since thyroid hormones are essential for the development of the fetal nervous system, etc., hypothyroidism should be taken seriously during pregnancy. Effects of hypothyroidism on pregnancy 1. Hypothyroidism in pregnant women is harmful to the fetus Adequate maternal thyroid hormones during pregnancy are necessary to ensure the health of the mother and her offspring. Hypothyroidism in mothers can affect the fetus’ neurological differentiation and development, and has a significant impact on the IQ of the child. The combination of maternal hypothyroidism and iodine deficiency can cause the fetal cortex, which is responsible for speech, hearing, movement and intelligence, to fail to fully differentiate and develop during development, resulting in slow growth, slow reaction time, dumb faces, and in some cases, deafness or mental disorders, called cretinism. Hypothyroidism during pregnancy can cause fetal miscarriage, stillbirth, hypothyroidism, impaired growth, goiter, and irreversible damage to IQ and nervous system. Patients with hypothyroidism have reduced fertility and are less likely to become pregnant and to miscarry if they do become pregnant. Hypothyroidism in pregnancy is associated with gestational hypertension, placental abruption, spontaneous abortion, fetal distress, preterm delivery, and the occurrence of low birth weight babies. See Table 1. 2. Subclinical hypothyroidism (sub-hypothyroidism) also requires attention Sub-hypothyroidism is a subclinical state between normal thyroid function and clinical hypothyroidism, defined as peripheral blood thyroid hormone levels in the normal range but mildly elevated thyroid stimulating hormone (TSH) levels. The incidence of combined hypothyroidism in pregnancy is 1% to 2% and combined subclinical hypothyroidism is 2.5%. Subclinical hypothyroidism in pregnant women is accompanied by a number of obstetric complications such as spontaneous abortion, pre-eclampsia, miscarriage, placental abruption, preterm delivery and postpartum hemorrhage. Studies have shown that children of mothers with subclinical hypothyroidism during pregnancy have lower IQ and increased pregnancy complications, so thyroxine replacement therapy should be administered to women with subclinical hypothyroidism during pregnancy and childbearing years. Treatment and monitoring of hypothyroidism in pregnancy The normal TSH value during pregnancy should be lower than in non-pregnancy. The American Thyroid Association recommends that the normal TSH value should be 0.1-2.5 mU/L in early pregnancy and 0.2-3.0 mU/L in mid- and late pregnancy. A diagnosis of subclinical hypothyroidism in pregnancy can be made above 2.5-3.0 mU/L. The normal value of TSH in pregnancy currently reported in China is higher than the normal value proposed by the American Thyroid Association. The normal value of TSH in early pregnancy is around 0.1~4.0mU/L, which is the normal value in pregnancy established by our recommended hospital or region. Fetal development depends on adequate T4 levels in the mother, not T3 levels; therefore, levothyroxine sodium (L-T4) is the agent of choice for replacement therapy in pregnant women or women preparing for pregnancy with hypothyroidism or subclinical hypothyroidism. In women of childbearing age who are preparing for pregnancy with hypothyroidism or subclinical hypothyroidism, thyroxine replacement therapy should be administered to achieve a TSH <2.5 mU/L before preparing for pregnancy. Once a pregnant woman is diagnosed with hypothyroidism or subclinical hypothyroidism, exogenous L-T4 should be supplemented in adequate amounts to correct maternal thyroid hormone levels and to ensure maternal supply of thyroid hormones to the fetus. During pregnancy, the dose of L-T4 is usually increased by 30% to 50% compared to the non-pregnant state. When adjusting the L-T4 dose it is recommended that TSH and FT4 be measured every 4-6 weeks. if TSH and FT4 are within normal values for pregnancy, this can be extended to a repeat test every 6-8 weeks. Women with normal thyroid function and positive thyroid peroxidase antibodies are at risk for hypothyroidism and require regular TSH testing. Postpartum management and monitoring After delivery, the physiology of hypothyroid and subclinical hypothyroid pregnant women gradually returns to pre-pregnancy levels. At this time, the thyroid hormone dose can be reduced to the dose taken before pregnancy, and thyroid function can be rechecked after 4-6 weeks of dose reduction. In the postpartum period, the suppressed immune function gradually returns to normal, and immune dysfunction can easily occur during the recovery process. Even if the thyroid function was normal before and during pregnancy, postpartum is prone to postpartum thyroiditis or chronic lymphocytic thyroiditis. It can be said that the 3-6 months after delivery is the period of high incidence of immune dysfunction. At this time, thyroid function should be checked, and if thyroid function is abnormal, treatment should be adjusted; if thyroid function is normal, thyroid function should be rechecked 1 year after delivery. Breastfeeding is very safe for women with hypothyroidism, and it is completely possible to breastfeed. It is important to note that the dose of thyroid hormone supplementation should not be too much or too little, but should mimic the physiological dose so that the TSH is maintained in the normal range. Newborns and infants rely mainly on iodine in breast milk to synthesize thyroid hormones on their own, therefore, some experts recommend an increase in iodine intake of 50 μg per day for breastfeeding mothers compared to non-breastfeeding women.