ObjectiveTo evaluate the effect of levothyroxine therapy on pregnancy outcomes compared with placebo or no treatment in women without overt hypothyroidism with presence of thyroid peroxidase antibodies (TPOAb) and/or thyroglobulin antibodies (TgAb).DesignSystematic review and meta-analysis of randomised controlled trialsStudy eligibility criteriaPrespecified criteria for inclusion were: randomised trials of levothyroxine versus control (placebo or no treatment) among women with positive TPOAb or TgAb who were pregnant or considering conception.Data sourcesOvid MEDLINE, EMBASE, CINAHL, Cochrane Database of Systematic Reviews and Cochrane Central Register of Controlled Trials were searched from 1980 to 5 November 2020.Outcome measuresPrespecified data elements were extracted and where appropriate, meta-analyses were conducted. Main outcomes include pregnancy achieved, miscarriage, preterm delivery and live birth.Risk of bias assessmentCochrane Risk of Bias Tool for Quality Assessment of Randomised Controlled Trials.ResultsFrom 3023 citations, 79 citations were identified for full-text review. Of these, six trials (total of 2263 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 to 1.13), miscarriage (RR 0.93; 95% CI 0.76 to 1.14), preterm delivery (RR 0.66; 95% CI 0.39 to 1.10) or live births (RR 1.01; 95% CI 0.89 to 1.16) in thyroid autoimmune women treated with levothyroxine compared with controls. Sensitivity analyses of preterm birth identified study quality and timing of levothyroxine initiation as sources of heterogeneity.ConclusionsAmong pregnant women or women planning conception, with thyroid autoimmunity, there is a lack of evidence of benefit for levothyroxine use (moderate to high Grading of Recommendations, Assessment, Development and Evaluations). Recommendations to use levothyroxine in this setting need to be reconsidered.PROSPERO registration numberCRD42019130459.