Etiology and diagnosis: The most common cause of hypothyroidism in pregnancy is disease of the thyroid gland itself, also known as primary hypothyroidism (including autoimmune thyroiditis; prior overtreatment for thyroid disease; antithyroid medication and iodine deficiency) and, rarely, secondary lesions originating from the hypothalamic pituitary gland. There are no definitive data on the prevalence of hypothyroidism in pregnancy, but subclinical hypothyroidism is more common than previously recognized. Clinical hypothyroidism is defined as elevated TSH and decreased TT4/FT4 with clinical symptoms of hypothyroidism; subclinical hypothyroidism is defined as elevated TSH and normal TT4/FT4 with insignificant clinical symptoms of hypothyroidism; hypotensive T4emia is defined as normal TSH and decreased TT4/FT4 and may have no clinical symptoms of hypothyroidism. How to treat: Once it is determined that a pregnant woman has combined clinical hypothyroidism and subclinical hypothyroidism, start treatment immediately with levothyroxine tablets at a starting dose of 25-50ug/d, taken early in the morning on an empty stomach, increasing by 25-50ug every 1-2 weeks, with a maintenance dose of 50-100-200ug/d. Make TSH reach and maintain below 2.5 mU/L in early pregnancy (or below 3 mU/L in middle and late pregnancy). As serum TSH is an extremely sensitive indicator, serum TSH levels should be monitored to adjust the levothyroxine tablet dose, and if the L-T4 dose is adjusted, TSH should be measured every 2-4 weeks. After the TSH standard is reached after treatment or dose adjustment, TSH, FT4 and TT4 should be monitored every 4-6 weeks. Take thyroid tablets at least 4 hours apart from pregnancy supplements such as iron, calcium, soy foods, and vitamins to prevent them from forming compounds that can be detrimental to thyroid tablet absorption.