INTRODUCTION Early detection of congenital hypothyroidism (CH) prevents impaired cognitive and motor development at problems. To increase the screening program specificity, we defined reference intervals for serum TSH in newborns between 2 and 14 days of life. PATIENTS AND METHODS Newborn infants from our neonatal screening program between 2001 and 2007. Serum TSH was determined by electrochemiluminescence. There were excluded neonates who died before the first month of life or with less than 1500 grams at birth, samples with TSH <0,01 µU/ml., processed after 14 days of life and from other medical centers. Statistical tools: Student t-tests, measures of centrality and dispersion, and logarithmic transformations. RESULTS Studied population: 6772 newborn infants out of 7869, 50.49% were males. The male/ female ratio was similar during the 14 days. The mean age at testing was 5.85 days (± 2.58), and the mean of TSH value was 4.42 µU/ml (± 4.17). Inverse relationship was found between age and TSH (mean TSH at days 2 and 14 was 5.82 µU/ ml and 2.58 respectively). CONCLUSION We found statistical evidence on an inverse relationship between TSH and age in newborn infants with 14 days of age. Reference intervals for TSH contribute to a better assessment of thyroid status in neonates and to define an appropriate conduct for individual therapy.