Background and Aims: Unequivocal data is not yet available on alterations in plasma concentrations of thyroid hormones that may characterize small-for-gestational-age (SGA) newborns. We used data from screening for congenital hypothyroidism in a large population to evaluate the relationship between growth restriction and thyroid function in the first week of life. Materials and Methods: Subjects: 14,092 newborns (13,333 appropriate-for-gestational-age (AGA) and 759 SGA) screened for congenital hypothyroidism on the third day of life. The screening test measured plasma concentrations of TSH and T4 by fluoroimmunoenzyme assay. Results: Comparison of SGA and AGA newborns revealed lower serum concentrations of T4 in preterm and term SGA infants (p=0.0001), whereas concentrations of TSH were significantly higher only in term SGA infants (p=0.0001). T4 concentrations were positively correlated with gestational age in SGA and AGA groups, whereas TSH concentrations were only correlated with gestational age in the AGA group. 1.84% of SGA babies were recalled for TSH and T4 anomalies against only 0.93% of AGA newborns (p=0.01). Conclusion: SGA babies had a higher incidence of transient hypothyroidism and required accurate follow-up and close monitoring of thyroid function.