The aim of this study was to assess the incidence of obstetric and neonatal complications in pregnant women with "normal" thyroid-stimulating hormone (TSH) levels in the first trimester (group A) and to compare them with those with "slightly elevated" TSH (SET) levels treated with levothyroxine (group B2) or not treated (group B1). A total of 2375 women who had been performed laboratory tests in their first trimester of pregnancy were detected at our hospital between April 2015 and August 2017. Of these, 469 patients with SET were prospectively detected and randomized to groups B1 (227) and B2 (242). They were monitored prospectively until 6 months after delivery. Data of the control group (n=1906, group A) were retrospectively reviewed. A total of 1745 women were analyzed. Variables assessed included demographic and clinical characteristics and complications of pregnancy and delivery. A, B1, and B2 had similar clinical characteristics. There were no statistically significant differences in complications between the three groups during pregnancy, except in that natural deliveries were more common in group A as compared to group B1 (76.8% vs. 68.7%, p 0.017) and group B2 (66.3%), p<0.002). There were more induced deliveries in groups B1 (35.8%), and B2 (36.2%) than in group A (18.4%), p<0.01. Although the recommended TSH level was achieved in the second and third trimesters, no benefit could be found of treatment of SET. Although there were less natural deliveries and more induced deliveries in patients with SET, treatment with levothyroxine could not reverse this situation, despite achievement of levels considered appropriate in the second and third trimester. Copyright © 2019 SEEN y SED. Publicado por Elsevier España, S.L.U. All rights reserved.