According to domestic and international data, the prevalence of combined hyperthyroidism in pregnancy is 0.02-0.2%, combined clinical hypothyroidism is 0.6%, combined subclinical hypothyroidism is 5.27%, and combined low T4 blood is 2.15%. Therefore, the number of patients with abnormal thyroid function in pregnancy is still very high, but it is still controversial whether all patients need to be tested for thyroid function before pregnancy. It is generally accepted that pregnant women with risk factors should have their thyroid function checked: personal history of thyroid disease; family history of thyroid disease; enlarged thyroid; positive thyroid antibodies with signs and symptoms suggestive of hyperthyroidism or hypothyroidism; type 1 diabetes; other autoimmune diseases; infertility; history of miscarriage and preterm delivery; history of head and neck radiation therapy and ten other conditions. 1. Is there a situation where hyperthyroidism does not exist before pregnancy, but occurs during pregnancy and continues? If hyperthyroidism occurs during pregnancy without hyperthyroidism before pregnancy, there are two cases: the first case is that the placenta produces a large amount of chorionic gonadotropin (hCG) in early pregnancy, which has the activity of thyroid stimulating hormone (TSH) and stimulates the thyroid gland, causing the serum thyroid hormone to rise and TSH to be suppressed, resulting in hyperthyroidism in pregnant women, which is called transient hyperthyroidism of pregnancy (THHG). This is called transient hyperemesis gravidarum (THHG), which is relatively mild and is often accompanied by severe pregnancy vomiting. The second type of hyperthyroidism occurs during pregnancy and is characterized by the same features as general hyperthyroidism, which is more severe and does not resolve easily. However, the latter condition is relatively rare, as the autoimmune disease is usually reduced or remitted due to the presence of immune tolerance during pregnancy. 2. What is the difference between transient hyperthyroidism in pregnancy and true hyperthyroidism? Transient hyperthyroidism in pregnancy occurs in the early stages of pregnancy and naturally remits in the middle and late stages of pregnancy, with mild symptoms, often accompanied by gastrointestinal symptoms such as loss of appetite, nausea, vomiting, etc. The thyroid gland is not enlarged and the thyroid autoantibodies are not high. The thyroid autoantibodies are often elevated, long-lasting and not easily relieved, and often require treatment. 3. What are the effects of hyperthyroidism on the mother and fetus? The incidence of spontaneous abortion and gestational hypertension increases in early and mid pregnancy with uncontrolled hyperthyroidism; the incidence of pre-eclampsia, eclampsia, congestive heart failure and hyperthyroidism crisis increases significantly in late pregnancy with uncontrolled hyperthyroidism; the incidence of stillbirth, preterm delivery, placental abruption and infection is much higher than in early and mid pregnancy with well-controlled hyperthyroidism. The main effects on the fetus are intrauterine growth retardation, prematurity, small full-term babies, congenital malformations, stillbirth, and premature closure of cranial suture. The incidence of fetal malformation in untreated hyperthyroidism is 6%, compared to 1.7% in treated hyperthyroidism, and only 0.2% in normal pregnancies. 4. Some patients start to show hyperthyroidism during pregnancy, but later the hyperthyroidism is relieved and recurs after delivery. Hyperthyroidism in pregnancy is characterized by an increase in hyperthyroidism in early pregnancy, a remission in the middle and late pregnancy, and a recurrence of hyperthyroidism after delivery. The beginning of hyperthyroidism in pregnancy may be related to the increase of serum hCG concentration in early pregnancy, because hCG has TSH-like effect. In the middle and late pregnancy, with the emergence of immune tolerance, decrease of TSAb titer, increase of serum TBG and decrease of iodine available in maternal thyroid, hyperthyroidism will be reduced or improved, or even become hypothyroidism. In the postpartum period, most of the remitted hyperthyroidism will recur as the immune tolerance is lifted.