Hyperthyroidism (hyperthyroidism) has a tremendous impact on the health of pregnant women and fetal development. Current guidelines recommend treating hyperthyroidism with propylthiouracil (PTU) in early pregnancy and switching to tapazole (MMI)/carbimazole (CMZ) in late pregnancy. However, a Danish study states that antithyroid drugs should be avoided in early pregnancy to reduce fetal birth defects. Therefore, doctors need to be extra cautious when treating pregnant women with antithyroid drugs. The researchers enrolled 30,000 participants on antithyroid drugs from the Danish Medical Birth Registry, the National Prescription Register, and the National Hospital Register. The participants included the general population and pregnant women. The data showed that the majority of patients used MMI/CMZ (27,281 cases) and only 5,895 cases applied PTU. The incidence of MMI-related adverse reactions in the general population was twice that of PTU-related adverse reactions, which may be related to the fact that the application rate of MMI was approximately five times that of PTU. The incidence of granulocyte deficiency was higher in those on PTU compared to those on MMI/CMZ (0.27% versus 0.11%, P=0.02). However, the difference in the incidence of liver failure was not statistically significant (0.05% and 0.03%, P=0.40). Further data showed that the incidence of antithyroid drug-associated granulocyte deficiency and liver failure was lower in pregnant women than in the general population, with only 5 cases of antithyroid drug-associated granulocyte deficiency and liver failure in each of the 10,000 pregnant participants. Among the 10,000 participants in the general population, there were 16 cases and 3 cases, respectively. A total of 75 birth defects associated with antithyroid drug use during pregnancy occurred in 2206 pregnant women on antithyroid drugs (340/10,000). The researchers recommend that women taking antithyroid drugs should take an early pregnancy test, inform their doctor if the test is positive, and not continue taking antithyroid drugs. If a doctor determines that a pregnant woman is in remission from hyperthyroidism, she should be closely monitored and receive weekly thyroid function tests until mid-pregnancy.