It is a common saying among Chinese parents that they should not let their children lose at the starting line. From the beginning of pregnancy, mothers-to-be start taking various brain development supplements such as deep-sea fish oil in the hope of giving birth to a smart and intelligent baby. However, mothers-to-be may not know that if hypothyroidism occurs during pregnancy, or to a lesser extent subclinical hypothyroidism, it may adversely affect the baby’s brain development, and if left untreated, the baby may really lose out at the starting line. During pregnancy, the obstetrician will ask the pregnant woman to undergo many tests and will pay great attention to certain diseases that occur during pregnancy, such as gestational hypertension and gestational diabetes, and will give active treatment. However, the diagnosis of thyroid disorders during pregnancy is not as important, and there are many misconceptions about treatment. There are many reasons for this, such as some maternal and child health care centers do not carry out thyroid function measurement, especially in rural areas, or they have done the thyroid function test but cannot make an accurate evaluation of the results, which delays the treatment; some hospitals put the thyroid function test in the middle or late pregnancy, and although the diagnosis of hypothyroidism is confirmed, the best time for treatment is missed. Therefore, this article describes the dangers of hypothyroidism during pregnancy and the problems in the treatment process, so that mothers-to-be can have a better understanding of the disease. Generally speaking, we recommend that mothers-to-be pay attention to thyroid function tests during the preparation stage, and if hypothyroidism or subclinical hypothyroidism already exists, they should be treated promptly and try to get the indicators back to normal before preparing for pregnancy. Of course, even if the pre-pregnancy thyroid function is normal, hypothyroidism or subclinical hypothyroidism may still occur during pregnancy, especially if you are over 30 years old, have other autoimmune diseases, have a family history or past history of thyroid disease (such as hyperthyroidism or Hashimoto’s thyroiditis), or are persistently thyroid peroxidase antibody (TPOAb) positive, the chance of hypothyroidism during pregnancy is significantly higher than others. Therefore, it is important to pay more attention to it and strengthen the testing of thyroid function during pregnancy. How can we determine the presence of hypothyroidism or suboptimal hypothyroidism during pregnancy based on the results of thyroid function tests? The most important thing to determine is the results of thyroid function tests during pregnancy. Due to changes in hormone levels during pregnancy, certain thyroid tests may be affected differently than during non-pregnancy. In general, the most sensitive indicator of hypothyroidism during pregnancy is the serum thyroid stimulating hormone (TSH) measurement, and in 2011 the American Thyroid Association suggested that if the TSH measurement is between 2.5-10.0 mIU/L (early pregnancy) or 3.0-10.0 mIU/L (mid- to late-pregnancy) and the FT4 level is decreased, or if the TSH level is >10.0 mIU/L regardless of the FT4 level, then the TSH level may be decreased. If the TSH level is >10.0mIU/L, the diagnosis of hypothyroidism in pregnancy can be made. If TSH is between the above ranges but FT4 levels are normal, then subclinical hypothyroidism can be diagnosed. Since this data is the result of a study on pregnant women in the United States, it is not entirely suitable for Chinese mothers-to-be. Moreover, since China is a vast country and the nutritional status of pregnant women varies from place to place, the Guidelines for the Diagnosis and Treatment of Thyroid Diseases in Pregnancy and the Postpartum Period formulated in China in 2012 clearly suggest the need to establish a pregnancy-specific TSH reference range for different iodine nutrition regions, and if the measured TSH value is greater than the pregnancy-specific If the measured TSH value is greater than the upper limit of the pregnancy-specific TSH reference value and the FT4 level is decreased, the diagnosis is hypothyroidism in pregnancy; if the FT4 is in the normal range and the measured TSH value is higher than the upper limit of the reference value, the diagnosis is subclinical hypothyroidism. It should be noted that the first trimester of pregnancy is a critical period for the development of the baby’s brain, and hypothyroidism can have very serious effects at this time. What are the risks of hypothyroidism in pregnancy or subclinical hypothyroidism to the baby? Hypothyroidism in pregnancy is harmful in three ways: First, hypothyroid pregnant women are more likely to suffer from miscarriage, anemia, hypertension, placental abruption, postpartum hemorrhage and other obstetric complications. Secondly, untreated hypothyroidism in pregnancy may lead to increased incidence of preterm delivery, low birth weight, neonatal respiratory distress syndrome, and increased risk of embryonic and perinatal mortality. Finally, and especially importantly, thyroid hormones play an important role in promoting fetal brain development, and deficiency can have serious adverse effects on brain development. Some foreign authors have found that the IQ (intelligence quotient) scores of the offspring of untreated hypothyroidism in pregnancy are lower than the average of the offspring of normal pregnant women, and that the offspring have 8-10 points lower behavioral cognitive ability scores. Both domestic and international scholars now agree that hypothyroidism in pregnancy must be treated as soon as possible. In China, the prevalence of subclinical hypothyroidism during pregnancy is much higher than that of hypothyroidism. A survey of pregnant women in Shenyang showed that the prevalence of subclinical hypothyroidism in pregnancy was 5.4%, which is a very high percentage. Some domestic scholars have found that the incidence of perinatal monitoring and neonatal respiratory distress syndrome increases with maternal subclinical hypothyroidism, and it also has a negative impact on the brain development of offspring. A follow-up study of offspring of mothers with subclinical hypothyroidism in early pregnancy found significantly lower IQ and motor scores at 20-30 months of age than offspring of mothers with normal thyroid function, but a number of studies found that the difference was not significant. Therefore, the need for active treatment of subclinical hypothyroidism is currently controversial both nationally and internationally. So how to treat hypothyroidism or subclinical hypothyroidism in pregnancy, and when to start treatment? First of all, it is recommended that mothers-to-be visit an endocrinology specialist at a tertiary care hospital, where the doctor will make a comprehensive assessment based on the pregnant woman’s TSH level, the number of weeks of pregnancy and the results of thyroid autoantibody testing. Generally speaking, hypothyroidism treatment requires supplementation with L-T4 (levothyroxine), especially for severely hypothyroid pregnant women, which must be used in adequate doses to correct hypothyroidism as soon as possible, with the specific dose determined by the doctor. Domestic and international studies have confirmed that the damaged brain function of the offspring can be restored after adequate L-T4 treatment for hypothyroid pregnant women, and mothers-to-be should not resist treatment with the wrong idea of not taking any medication during pregnancy. For subclinical hypothyroidism in pregnancy, if TOPAb is positive, L-T4 therapy is given, if TPOAb is negative, close observation can be made, and if there is a persistent increase in TSH or a decrease in FT4 level, L-T4 therapy is given. Because the first 12 weeks of pregnancy is a critical period for fetal brain development, and because the fetal thyroid gland is not yet mature, it cannot secrete thyroid hormone on its own and is therefore completely dependent on the mother’s supply. Domestic and international guidelines point out that early pregnancy is the best time for L-T4 treatment, and the later the treatment is given, the worse the treatment effect will be, but of course, it is necessary to adhere to the medication throughout pregnancy and not to stop it easily. In conclusion, hypothyroidism or hypothyroidism in pregnancy can have adverse effects on both the pregnant woman and the fetus, so mothers-to-be should be vigilant and have their nail function tested in early pregnancy.