The debate on thyroid screening during pregnancy continues.

The current controversy over recommendations for thyroid screening during pregnancy has its origins in the 1999 publication of an observational study based on thyroid function analysis of stored secondtrimester serum samples. In this study, 62 children of mothers identified with thyroid-stimulating hormone (TSH) levels greater than the 98th percentile were compared with 124 children of euthyroid-matched controls. Neurodevelopmental testing at 7–9 years of age revealed that children born to women with untreated thyroid dysfunction had lower mean IQ scores. Women included in this study had low mean serum-free thyroxine (T4) levels, and many would be considered to have had overt hypothyroidism. Despite that, this and other studies linking increases in preterm birth, placental abruption, and fetal death to subclinical thyroid dysfunction have been cited by professional organizations as the impetus for recommendations of routine prenatal screening for and treatment of subclinical hypothyroidism. In contrast, the American College of Obstetricians and Gynecologists has consistently recommended against universal screening and supported a risk-based approach until there is evidence of treatment benefit in women with subclinical hypothyroidism or their offspring. Importantly, women with subclinical hypothyroidism represent the majority of those who would be identified through routine TSH screening. Of note, the recently completed international multicenter Controlled Antenatal Thyroid Screening study did not demonstrate improvement in cognitive function of 3-year-old children born to women screened and treated for subclinical thyroid dysfunction during pregnancy. In contrast to subclinical thyroid dysfunction, it is well-established that untreated overt hypothyroidism during pregnancy is associated with adverse outcomes for both the mother and her fetus and that treatment mitigates many of these risks. Overt hypothyroidism is defined by an elevated TSH level and decreased free T4 level according to thresholds that vary by gestational age and population studied. If one were to set aside the debate surrounding routine identification and treatment of pregnant women with subclinical thyroid dysfunction, the remaining question is whether detecting and treating women with undiagnosed overt hypothyroidism provides sufficient rationale for universal screening during pregnancy. It has been suggested that targeted thyroid testing during pregnancy may miss approximately 30% of women with elevated TSH due to overt or subclinical hypothyroidism. However, a recent cost-effectiveness analysis suggests that a policy of universal screening to detect overt hypothyroidism during pregnancy falls below recommended U.S. cost-effectiveness thresholds. This month in Obstetrics & Gynecology, Granfors and colleagues (see page 10) assess the practice of targeted thyroid testing in a retrospective cohort of women whose blood was drawn at the time of routine ultrasound screening at 17–19 weeks of gestation and stored for later study. See related article on page 10.