Objective: Uncontrolled hypothyroidism has been associated with an increased risk of adverse pregnancy outcomes. We aimed to assess the effectiveness of increasing levothyroxine (LT4) dose on reducing the risk of adverse outcomes for pregnant women with TSH level greater than the recommended 1st trimester limit. Design, Patients, Measurements: We reviewed the electronic medical records of pregnant women evaluated from January 2011 to December 2013, who had history of LT4-treated hypothyroidism and were found to have TSH > 2·5 mIU/l in 1st trimester. Women were divided into two groups: group A – LT4 dose was increased within two weeks from the TSH test, group B – LT4 dose remained stable. We compared the frequency of pregnancy loss (primary outcome) and other prespecified pregnancy-related adverse outcomes between groups. Results: There were 85 women in group A (median TSH: 5·0, interquartile range 3·8–6·8 mIU/l) and 11 women in group B (median TSH: 4·5, interquartile range 3·2–4·9 mIU/l). The groups were not different in baseline clinical and socioeconomic characteristics. The mean interval between TSH test and LT4 dose increase was 4·5 (SD 4·6) days. Pregnancy loss was significantly lower in group A (2/85, 2·4%) vs group B (4/11, 36·4%) (P = 0·001). Other pregnancy-related adverse outcomes were similar between groups. Conclusions: Increasing LT4 dose for women with uncontrolled hypothyroidism in the 1st trimester of pregnancy was associated with a decreased risk of pregnancy loss. Given the limitations of our study, this association awaits further confirmation from larger studies.