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Euthyroid women with thyroid peroxidase antibodies are at excess risk for miscarriage and preterm birth, but whether exogenous levothyroxine is beneficial to such women remains unknown. Investigators in the U.K. conducted a multicenter trial in which 952 euthyroid women (age range, 16–40) with a history of miscarriage or infertility and seeking to conceive were randomized to 50-µg oral levothyroxine or placebo daily from before conception through the end of pregnancy. Euthyroid status was defined as serum thyrotropin 0.44–3.63 mIU/L and free thyroxine 10.0–21.0 pmol/L (0.78–1.63 ng/dL). Group assignments were balanced by age (<35 or ≥35), number of previous miscarriages, infertility treatment, and baseline thyrotropin concentration (≤2.5 or…