Objective The aims were to establish a gestational-age specific curve for serum total thyroxine (T4) levels and to compare pregnancy outcomes of euthyroid women with those identified to have subclinical hypothyroidism (SCH) defined by an elevated thyroid-stimulating hormone (TSH) level in conjunction with either total T4 or free T4 determinations. Study Design Over a 2.5 year period, serum thyroid analytes were measured in all women presenting for prenatal care. After exclusion of women with overt thyroid disorders, the normal distribution of serum total T4 levels were determined by quantile curves for those screened in the first 20 weeks and who were delivered of a singleton infant weighing at least 500 g. Pregnancy outcomes for women with an elevated TSH and normal total T4 concentrations were analyzed and compared with those of women identified to have SCH defined by normal free T4 levels. Results Of 17,298 women tested, serum total T4 increased into the second trimester and plateaued around 16 weeks. The upper threshold for total T4 ranged from 12.6 to 16.4 μg/dL, and the lower threshold ranged from 5.3 to 8.0 μg/dL. Women identified to have SCH defined by serum free T4, total T4, or both were at risk for preterm delivery (P =.007) and placental abruption (P =.013) when compared with euthyroid women. Conclusion When combined with elevated TSH levels, free or total T4 determinations are equally sensitive to identify women with SCH who are at increased risk for preterm birth and placental abruption when compared with euthyroid women. © 2014 Mosby, Inc. All rights reserved.