The purpose of the study was to evaluate pregnancy outcomes of hypothyroidism that were identified in a population-based prenatal screening program.

This is a secondary analysis of a prospective prenatal population-based study in which serum thyroid analytes were obtained from November 2000 to April 2003. Initial screening thresholds were intentionally inclusive (thyroid-stimulating hormone [TSH], >3.0 mU/L; free thyroxine, <0.9 ng/dL); those who screened positive were referred for confirmatory testing in a hospital-based laboratory. Hypothyroidism was identified and treated if TSH level was >4.5 mU/L and if fT4 level was <0.76 ng/dL. Perinatal outcomes in these women and those who screened positive but unconfirmed to have hypothyroidism were compared with women with euthyroidism. Outcomes were then analyzed according to initial TSH levels.

A total of 26,518 women completed initial screening: 24,584 women (93%) were euthyroid, and 284 women (1%) had abnormal initial values that suggested hypothyroidism. Of those referred, 232 women (82%) underwent repeat testing, and 47 women (0.2% initially screened) were confirmed to have hypothyroidism. Perinatal outcomes of women with treated overt hypothyroidism were similar to women with euthyroidism. Higher rates of pregnancy-related hypertension were identified in the 182 women with unconfirmed hypothyroidism when compared with women with euthyroidism (P < .001); however, this association was seen only in women with initial TSH >4.5 mU/L (adjusted odds ratio, 2.53; 95% confidence interval, 1.4–4.5).

The identification and treatment of overt hypothyroidism results in pregnancy outcomes similar to women with euthyroidism. Unconfirmed screening results suggestive of hypothyroidism portend pregnancy risks similar to women with subclinical hypothyroidism, specifically preeclampsia; however, this increased risk was seen only in women with initial TSH levels of >4.5 mU/L and suggests that this is a more clinically relevant threshold than 3.0 mU/L.