Treatment of pre-existing hyperthyroidism in pregnancy

Carbimazole therapy should have been switched to propylthiouracil before conception, as it is associated with less severe congenital abnormalities. If the woman is in the first trimester and is still taking carbimazole, switch to propylthiouracil.

The goal of treatment of hyperthyroidism in pregnancy is to maintain maternal euthyroidism, with the maternal serum T₄ concentration in the upper half of the trimester-specific reference range. Measure maternal serum TSH and T₄ concentrations every 4 to 6 weeks in a patient who is stable and euthyroid on treatment. More frequent testing is appropriate if the dose is adjusted or the patient is unstable. To prevent overtreatment and possible neonatal hypothyroidism or goitre, and to minimise the risk of fetal congenital abnormalities, use the lowest effective dose of antithyroid drug therapy.

Many women with Graves disease experience remission or improvement of disease during pregnancy, allowing antithyroid therapy to be progressively reduced and often stopped. However, the disease often relapses postpartum (see Postpartum thyroid dysfunction for more information).

If antithyroid therapy is still required during the second trimester, switch therapy back to carbimazole; the risk of fetal congenital abnormalities has passed by this time, and carbimazole has a lower risk of liver injury.

If hyperthyroidism is difficult to control (eg requiring large doses of carbimazole), surgery can be considered during pregnancy. Seek specialist advice.