Hypothyroidism in pregnancy refers to hypothyroidism that occurs during pregnancy, i.e., a condition in which the thyroid gland is unable to synthesize or secrete enough thyroxine to meet the body’s needs. Early pregnancy, i.e. the first 12 weeks of pregnancy, is the first rapid development period of fetal brain development, but at this time, the thyroid function of the fetus itself has not yet been established, and the thyroid hormone needed for fetal brain development is completely dependent on the mother’s supply. After 3 months, the fetus’ own thyroid gland has been able to synthesize thyroid hormone, but the amount is small, and still need to rely on the mother’s hormone to help. If the mother suffers from hypothyroidism in early pregnancy, it will affect the brain development of the fetus, and even cause irreversible damage, resulting in a 6-8 point drop in the IQ of the offspring. Therefore, pregnant women with hypothyroidism should not stop taking their medication, and should make dosage adjustments under the guidance of a doctor. Taking proper medication will not affect the fetus, on the contrary, if not treated, the child’s insufficient supply of thyroid hormones will affect the development of the fetus, and in severe cases, will suffer from cretinism. Before preparing for pregnancy, it is important to check the indicators of thyroid function and thyroid autoimmune antibodies. If the serum TSH is between 0.3-2.5 mIU/L, it is the best time to get pregnant. After pregnancy, thyroid function and antibodies should be checked regularly. For patients who have been diagnosed with hypothyroidism before pregnancy, it is best to control the serum TSH below 2.5 mIU /L by taking medication before considering pregnancy. After pregnancy, individualized dosage adjustments should be made under medical supervision. Patients who are diagnosed with hypothyroidism after pregnancy should take medication immediately and control their serum TSH below 2.5 mIU /L. The earlier the TSH is reached, the less impact hypothyroidism in pregnancy will have on the brain development of the fetus. Mothers-to-be suffering from hypothyroidism rely entirely on the levothyroxine sodium tablets they take to maintain their own thyroid function normally and at the same time to provide the thyroxine (T4) needed for fetal brain development. Therefore, levothyroxine sodium tablets must not be discontinued during pregnancy, but on the contrary, the dose must be increased appropriately because the demand for levothyroxine increases by 30%-50% after pregnancy compared with that before pregnancy. After delivery, the dosage of levothyroxine sodium tablets can be restored to the pre-pregnancy dosage under medical supervision. Thyroid hormone is a hormone normally produced by our body, and exogenous thyroxine is taken only to supplement the part of the body that is otherwise deficient. Taking the correct dose of exogenous thyroxine (e.g. levothyroxine sodium tablets) is perfectly safe and does not affect pregnancy or breastfeeding. The best alternative medicine for hypothyroidism is levothyroxine sodium tablets (L-T4), which should be taken on an empty stomach. It is best not to eat soy products for 4 hours after taking them, and it is best not to take medicines containing various vitamins and minerals at the same time in order to avoid affecting their absorption. Early treatment and standardized use of levothyroxine therapy is vital for both mother and child.