Treatment of pre-existing hypothyroidism before conception and during pregnancy

If a woman taking levothyroxine for hypothyroidism is planning to become pregnant, optimise the levothyroxine dose to achieve a preconception TSH concentration below 2.5 milliunits/L.

During a healthy pregnancy, thyroid hormone production increases by up to 50%. To avoid undertreatment, particularly in the first trimester, women already taking levothyroxine before pregnancy usually require a 25 to 30% dose increase at 4 to 6 weeks gestation. Consider testing serum TSH concentration approximately every 4 to 6 weeks during the first trimester, and adjusting the levothyroxine dose accordingly. If the serum TSH concentration is stable, testing again at around 30 weeks gestation is sufficient.

After delivery, reduce the dose of thyroxine to the maintenance dose used before pregnancy and measure the serum TSH concentration approximately 6 weeks postpartum.

Women with hypothyroidism are at increased risk of Postpartum thyroid dysfunction.