Background: Different and conflicting data have been published concerning thyrotropin (TSH) levels among small-for-gestational-age (SGA) and appropriate-for-gestational-age (AGA) newborns. The hypothesis of this study was that SGA preterm infants have higher TSH levels than those who are not underweight do. Methods: This cross-sectional study analyzed the TSH levels of all preterm newborns with a negative congenital hypothyroidism screening result (TSH <7.5 mu IU/mL). Secondary variables were sex, birth weight (SGA, AGA), days of life at blood extraction, maternal origin, gestational age, and being a twin or not. Two multiple linear regression models were constructed comparing TSH levels in SGA and AGA or the z-score for birth weight and the remaining variables. Results: A sample including 5819 preterm infants was obtained: 53.8% male, 23.3% twins, and 3.3% SGA. TSH concentrations were 2.16+/-2.0 mu IU/mL for the SGA infants and 1.80+/-1.5 mu IU/mL for the AGA infants (p = 0.012), with a negative correlation (p < 0.001) between TSH levels and the z-score for the weight of the newborn. The multivariate analysis comparing TSH levels between SGA and AGA gave the following: SGA (B=0.46, p < 0.001), Latin American mother (B=-0.16, p = 0.029), days of life at blood extraction (B=-0.26, p < 0.001), and gestational age <= 28 weeks (B=-0.56, p < 0.001). Using the z-score for the weight, the associations were: maternal origin North Africa (B=0.19, p = 0.042), days of life at blood extraction (B=-0.27, p < 0.001), gestational age <= 28 weeks (B = -0.55, p < 0.001), and z-score for weight (B=-0.12, p < 0.001). Conclusions: Our multivariate analysis suggests that TSH concentrations are higher in SGA infants than they are in AGA infants, and this should be taken into account when establishing a reference interval appropriate for this population. The clinical relevance remains unknown, but lines of research are opened that may allow a better understanding of the long-term morbidities in these newborns.