Thyroid disease in pregnancy: (Women's Health Series).

Pregnancy is a state of many hormonal changes that can make interpretation of thyroid function tests difficult. Measuring trimester-specific reference values of thyrotropin and free thyroxine is recommended. Because overt maternal hypothyroidism negatively affects the fetus, timely recognition and treatment are important. Women taking levothyroxine prepregnancy require a ≤50% dose increase during pregnancy. Hyperthyroidism can result from excessive human chorionic gonadotropin or Graves disease. Radioactive scanning should be avoided during pregnancy. Antithyroidal drug therapy should consist of propylthiouracil during the first trimester and methimazole thereafter. If indicated, beta blockers can be administered under obstetrical supervision. Iodine deficiency is a known goitrogen and stimulus for thyroid nodular growth. Thyroid nodules may enlarge, but the incidence of thyroid cancer is not increased during pregnancy. Suspicious nodules should be biopsied and, if necessary, removed during the second trimester; otherwise, follow-up can safely be conducted postpartum. Thyroid-stimulating hormone suppression for any preexisting thyroid cancer or suspicious nodules should achieve free or total T4 in the upper normal range for pregnancy. Postpartum thyroiditis occurs more frequently in antithyroid peroxidase-positive women, who should be screened by measuring serum thyrotropin at 6 to 12 weeks' gestation and at 3 and 6 months postpartum.