Both propylthiouracil and methimazole are used to treat hyperthyroidism. Both propylthiouracil and methimazole inhibit thyroid hormone synthesis, inhibit iodine activation, prevent the binding of activated iodine to tyrosine, and block the peroxidase system in the thyroid gland, thereby preventing the synthesis of thyroxine. Propylthiouracil also inhibits the conversion of T4 to T3 in peripheral tissues and reduces the activity of thyroid hormones. Both propylthiouracil and methimazole have certain side effects, including liver damage and granulocytopenia, as well as small vessel vasculitis and skin irritation with propylthiouracil. Methimazole is the first choice for antihyperthyroidism. Propylthiouracil is preferred during pregnancy and preparation. Propylthiouracil rarely crosses the placental barrier and has less effect on the fetus, and is safer throughout pregnancy. Liver function and blood count should be reviewed regularly when using antihyperthyroid drugs to prevent liver damage and granulocytopenia, and to prevent serious infections or liver failure.