Context: Mild maternal thyroid dysfunction during early pregnancy is associated with poor neurodevelopment in affected offspring. Most studies are population based or are smaller populations of term/late preterm infants. No studies were found that focused on more preterm infants. Objective: Our objective was to describe the relationship between mild maternal thyroid dysfunction at delivery of infants born ≤34 wk and neurodevelopment at 5.5 yr. Design: The study designwasfollow-up ofwomenandchildren recruited in Scotlandbetween1998 and 2001. Main Outcome: We evaluated delivery levels of maternal TSH, free T4 (FT4), and T4 and the association with McCarthy Scale scores adjusted for 26 confounders of neurodevelopment. Results: Maternal serum levels and McCarthy scores were available for 143 women and 166 children. After adjustment for confounders, there were significant 3.2, 2.1, and 1.8 point decrements, respectively, in general cognitive index, verbal subscale, and the perceptual performance subscale for each milliunit per liter increment in maternal TSH. Maternal FT4 levels were variably associated with neurodevelopment. After adjustment, significant associations were found for the general cognitive index, motor scale, and quantitative subscale; each picomole per liter decrease in FT4 was associated with an increase of 1.5, 1.7, and 0.9 points, respectively. Maternal T4 levels showed little relationship with neurodevelopment. None of the women in this analysis had overt hypothyroidism, but mild hypothyroidism was evident in 27%; thyroglobulin antibody (TgAb) was ≥ 40 U/ml in 28% of the women. Conclusions: Higher maternal levels ofTSHat delivery of infants born pretermwereassociated with significantly lower scores on the general cognitive index at 5.5 yr. Copyright