Background: Thyroid autoimmunity in pregnant women has been associated with negative outcomes. Objective: To evaluate the effect of levothyroxine therapy on pregnancy outcomes compared with placebo or no treatment in women without overt hypothyroidism who are TPOAb and/or TgAb-positive. Search Strategy: Ovid MEDLINE, EMBASE, CINAHL, Cochrane Database of Systematic Reviews, and Cochrane Central Register of Controlled Trials were searched from 1980 to April 17, 2019. Selection Criteria: Pre-specified criteria for inclusion were: randomized trials of levothyroxine versus control (placebo or no treatment) among women with positive TPOAb or TgAb who were pregnant or considering conception. Data Collection and Analysis: Pre-specified data elements were extracted and where appropriate, meta-analyses were conducted. Main outcomes include pregnancy achieved, miscarriage, preterm delivery and live birth. Main Results: From 2,812 citations, 79 citations were identified for full text review. Of these, six trials (total of 2,263 women) were included for qualitative and quantitative analyses. Risk of bias was deemed low for only one trial. There was no significant difference in the relative risk (RR) of pregnancy achieved (RR 1.03; 95% CI 0.93, 1.13), miscarriage (RR 0.93; 95% CI 0.76, 1.14), preterm delivery (RR 0.66; 95% CI 0.39, 1.10), or live births (RR 1.01; 95% CI 0.89, 1.16) in thyroid autoimmune women treated with levothyroxine compared to controls. Conclusion: Among pregnant women or women planning conception, with thyroid autoimmunity, there is a lack of evidence of benefit for levothyroxine use. Recommendations to use levothyroxine in this setting need to be reconsidered.