CONTEXT: Thyroid dysfunction is associated with adverse obstetric outcomes, but there is limited information on pregnancy outcomes in women established on levothyroxine. OBJECTIVE: The objective of the study was to determine the relationship between TSH levels and pregnancy outcomes in levothyroxine-treated women in a large community-based database. DESIGN: This was a historical cohort analysis. PATIENTS: Individuals with a first prescription of levothyroxine from 2001 through 2009 (n = 55 501) were identified from the UK General Practice Research Database (population 5 million). Of these, we identified 7978 women of child-bearing age (18-45 y) and 1013 pregnancies in which levothyroxine had been initiated at least 6 months before conception. MAIN OUTCOME MEASURES: TSH, miscarriage/delivery status, and obstetric outcomes were measured. RESULTS: Forty-six percent of levothyroxine-treated women aged 18-45 years had a TSH level greater than 2.5 mU/L (recommended upper level in the first trimester). Among pregnant women who had their TSH measured in the first trimester, 62.8% had a TSH level greater than 2.5 mU/L, with 7.4% greater than 10 mU/L. Women with TSH greater than 2.5 mU/L in the first trimester had an increased risk of miscarriage compared with women with TSH 0.2-2.5 mU/L after adjusting for age, year of pregnancy, diabetes, and social class (P = .008). The risk of miscarriage was increased in women with TSH 4.51-10 mU/L [odds ratio (OR) 1.80, 95% confidence interval (CI) 1.03, 3.14)] and TSH greater than 10 mU/L (OR 3.95, 95% CI 1.87, 8.37) but not with TSH 2.51-4.5 mU/L (OR 1.09, 95% CI 0.61, 1.93). CONCLUSIONS: The majority of levothyroxine-treated women have early gestational TSH levels above the recommended targets (>2.5 mU/L) with a strong risk of miscarriage at levels exceeding 4.5 mU/L. There is an urgent need to improve the adequacy of thyroid hormone replacement in early pregnancy.