Thyroid screening in pregnancy.

The adverse impact of overt hypothyroidism that complicates pregnancy outcomes is well-established and not debated. For more than a decade, however, endocrinologists and obstetricians have been debating whether screening for subclinical thyroid disorders during pregnancy should be routine or should continue to be based on symptoms and risk factors. Several observational studies have suggested that offspring of women with asymptomatic thyroid dysfunction were at increased risk for impaired neurodevelopment. Other studies have suggested that pregnant women with subclinical thyroid disease, particularly those identified with an elevated thyroid-stimulating hormone (TSH) level may be at increased risk for pregnancy complications such as fetal death, preterm birth, or placental abruption. These data have prompted both obstetric and endocrinologic professional societies to draft recommendations regarding screening for thyroid disease during pregnancy, some of which are not entirely based on available evidence. The prevalence of overt thyroid disease is estimated to be 1-2 per 1000 pregnancies and historically has not been considered high enough to justify routine screening. Lower TSH thresholds (>2.5 mU/L) for the diagnosis of hypothyroidism have been promoted, and women with subclinical thyroid dysfunction commonly are included in estimates of thyroid disease during pregnancy, both of which exaggerate prevalence rates. The most compelling recent evidence on this issue has come from the Controlled Antenatal Thyroid Screening trial. After almost 22,000 pregnant women were screened for either isolated high TSH or isolated low free thyroxine level, 390 children of treated women with either diagnosis were compared with 404 children of similar women who were not treated during pregnancy. Treatment had no effect on mean offspring IQ at age 3 years or the number of children with an IQ <85. Authors of this landmark study concluded that antenatal screening and maternal treatment for women with subclinical thyroid dysfunction did not result in improved cognitive function. An ongoing intervention trial conducted by the Eunice Kennedy-Shriver National Institute of Child Health and Human Development's Maternal-Fetal Medicine Units Network will provide further clarity to this important question. In the interim, the debating authors have concluded, after careful review of the currently published literature, that routine screening for subclinical thyroid dysfunction during pregnancy is not currently warranted at this time.