Management of Thyroid Disease During and After Pregnancy

The Endocrine Society has revised its 2007 guidelines for managing thyroid disease during pregnancy and postpartum. Highlights of the updated guidelines are as follows:

• Universal screening for thyroid dysfunction before pregnancy in healthy women is not recommended.

• The committee could not agree whether all newly pregnant women, or only those at high risk for thyroid disease (see table), should be screened for thyroid-stimulating hormone (TSH) abnormalities.

• Once-daily prenatal vitamins should contain 150–200-µg iodine in the form of potassium iodide or iodate and preferably should begin before conception (note that not all prenatal vitamins contain the recommended amount of iodine; New Engl J Med 2009; 360:939).

• Breast-feeding women should take 250-µg iodine daily to ensure that breast milk provides 100 µg to the infant daily.

• Some free thyroxine (T₄) assays are methodologically problematic during pregnancy, necessitating caution in the context of both hypothyroidism and hyperthyroidism; therefore, the nonpregnant total T₄ range (typically 5–12 µg/dL or 50–150 nmol/L) can be adapted for use during the second and third trimesters by multiplying this range by 1.5. The free T₄ index (“adjusted T₄”) also seems reliable during pregnancy.

• Antithyroid drug therapy remains the first-line choice for hyperthyroidism in pregnancy. Because propylthiouracil can rarely be associated with severe maternal liver toxicity, methimazole should be considered as an alternative after the first trimester (the two agents are equally efficacious, but first-trimester fetal exposure to methimazole has been associated with congenital abnormalities).

• Because thyroid receptor antibodies freely cross the placenta and can act on the fetal thyroid, maternal antibodies should be measured before 22 weeks' gestation in women with Graves disease; histories of Graves disease managed with radioactive iodine or thyroidectomy before pregnancy; previous neonates with Graves disease; or previously elevated thyroid receptor antibodies.

• In women who have elevated thyroid receptor antibodies or are receiving antithyroid drugs, the possibility of fetal thyroid dysfunction (as evidenced by thyroid enlargement, growth restriction, hydrops, goiter, advanced bone age, tachycardia, or cardiac failure) should be assessed with ultrasound at 18 to 22 weeks and thereafter every 4 to 6 weeks or as clinically indicated.

• Although maternal thyroid antibodies have been associated with pregnancy loss, universal screening and possible treatment with exogenous T₄ cannot be recommended because evidence is insufficient that such treatment lowers miscarriage rates.

• Thyroid nodules identified during pregnancy should be evaluated (although those identified during the last 6 weeks may be evaluated after delivery). Fine-needle aspiration should be performed for predominantly solid ≥1-cm nodules, for complex 1.5–2-cm nodules, and for 5-mm–1.0-cm nodules in women with high-risk histories or suspicious ultrasound findings.

• Radioactive iodine should not be given to breast-feeding women until ≥4 weeks after weaning; pregnancy should be avoided for 6 to 12 months in women with thyroid cancer who have been treated with radioactive iodine.