Thyroid disease is relatively common in pregnancy, and in their overt states, both hypothyroidism and hyperthyroidism have been consistently associated with adverse pregnancy outcomes. Fortunately, these risks are mitigated by adequate treatment. It is essential to understand the gestational age-specific effects that pregnancy has on thyroid function tests. Failure to recognize these physiologic alterations can result in misclassification or misdiagnosis. Women who have a history of a thyroid disorder or symptoms that suggest thyroid dysfunction should be screened with thyroid function tests. Screening asymptomatic pregnant women, however, is of unproven benefit and is not currently recommended.
Overt hypothyroidism is defined by an elevated serum TSH level with a depressed FT4 level. The condition in pregnancy has consistently been associated with an increase in complications such as spontaneous abortion, preterm birth, preeclampsia, placental abruption, and impaired neuropsychological development in the offspring. The most common etiology is Hashimoto (autoimmune) thyroiditis. Many of the symptoms of hypothyroidism mimic those of normal pregnancy, making its clinical identification difficult. Initial treatment is empiric with oral levothyroxine started at 75–100 mcg/day. Thyroid function tests can be repeated at 4–6 weeks and the dose adjusted as necessary with the goal of normalizing the TSH level (preferably to a trimester-specific gestational reference range). An increase in the dose of levothyroxine may be required in the second and third trimesters.
Subclinical hypothyroidism is defined as an increased serum TSH with a normal FT4 level. Although some studies have found associations with untoward pregnancy outcomes such as miscarriage, preterm birth, and preeclampsia, others have failed to confirm these findings. There is currently no evidence, however, that identification and treatment of subclinical hypothyroidism will prevent any of these outcomes. Early observational studies also suggested that cognitive function was impaired in offspring of women with untreated subclinical hypothyroidism. Data from an NIH-sponsored Maternal-Fetal Medicine Units Network randomized, controlled trial demonstrated no improvement in cognitive function of 5-year-olds born to women screened and treated for subclinical hypothyroidism. For these reasons, the American College of Obstetricians and Gynecologists and the American Association of Clinical Endocrinologists recommend against universal screening for thyroid disease in pregnancy.
Overt hyperthyroidism, defined as excessive production of thyroxine with a depressed (usually undetectable) serum TSH level, is also associated with increased risks in pregnancy. Spontaneous abortion, preterm birth, preeclampsia, and maternal heart failure occur with increased frequency with untreated thyrotoxicosis. Thyroid storm, although rare, can be a life-threatening complication. Medical treatment of thyrotoxicosis is usually accomplished with the antithyroid drugs propylthiouracil or methimazole. Although teratogenicity has not been clearly established, in utero exposure to methimazole has been associated with aplasia cutis and choanal and esophageal atresia in the offspring of pregnancies so treated. Propylthiouracil is not believed to be teratogenic, but it has been associated with the rare complications of hepatotoxicity and agranulocytosis. Recommendations by the American Thyroid Association are to treat with propylthiouracil in the first trimester and ...