Because hypothyroidism during pregnancy can impair fetal cognitive development and increases the risk of fetal death, pregnant women with hypothyroidism are usually advised to increase their levothyroxine dosage. But how soon, and by how much?

To investigate the issue, researchers measured thyroid function, human chorionic gonadotropin, and estradiol levels before and throughout the pregnancies of 19 women (20 pregnancies resulting in 17 full-term births). They increased women's doses of levothyroxine to maintain thyrotropin levels at preconception values throughout pregnancy. They found that a higher levothyroxine dose was needed in 17 pregnancies. The average drug requirement rose 47% during the first half of pregnancy and plateaued by week 16 of gestation. Women needed the higher dose until delivery.

Based on these findings, the researchers recommend that women with hypothyroidism should have their levothyroxine dose increased by about 30% as soon as pregnancy is confirmed. Their serum thyrotropin levels should then be monitored throughout pregnancy and the levothyroxine dose adjusted as indicated.

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