The maternal and fetal demand for thyroid hormone increases during pregnancy, and only free thyroxine can provide thyroxine to the fetus through the placenta. The increase in maternal requirement for thyroid hormone occurs between the 4th and 6th week of gestation and gradually rises until it reaches a steady state at the 20th week of gestation. It is maintained until delivery. The L-T4 replacement dose needs to be increased by approximately 25%-30% after pregnancy in clinically hypothyroid women. The dose should be adjusted according to the serum TSH therapeutic goals mentioned above (remember to always be under medical supervision). It is recommended that once a hypothyroid patient is found to be pregnant, the thyroid function should be rechecked at the first time. Many patients already have a thyroxine deficiency at this time, and the first dose adjustment should be made according to the thyroid function under the guidance of the doctor. When there is a problem with thyroid function, it is recommended to check and adjust the dose once in half a month to quickly adjust the thyroid function to the appropriate range. The frequency of monitoring thyroid function in the first half of pregnancy (1-20 weeks) in clinically hypothyroid pregnant women is once every 4 weeks. Serum thyroid function indicators should be tested once at 26-32 weeks of gestation.