Primary hypothyroidism is caused by a decrease in thyroid hormone secretion by the thyroid gland. Thyroid function tests show a decrease in T3 and T4 and an increase in TSH. The treatment of primary hypothyroidism includes the following points: First, a low iodine diet, too much iodine can easily cause further damage to the thyroid gland and aggravate the symptoms of primary hypothyroidism. The dose of replacement therapy is determined by the degree of hypothyroidism. Third, the thyroid function is reviewed regularly during the treatment to adjust the drug dose and maintain the normal range of TSH. Fourth, women with primary hypothyroidism who become pregnant need to increase the dose of medication to maintain TSH between 2.5-3.0 mU/L during pregnancy to avoid affecting fetal development.