Objective: To evaluate the seasonal differences in newborn TSH levels.Method: A restrospective study was performed using normal blood TSH measurements collected in a neonatal screening program for congenital hypothyroidism, during the extremes of temperature (summer/winter) in order to verify the seasonal influence on that exam. Blood samples were collected from baby feet on filter paper in health care public institutions in Rio de Janeiro city and TSH were analyzed by immunofluorimetric method.For this study we considered samples with TSH levels less then 20µU/ml. The factors age, birth weight and prematurity were all observed.Results: We evaluated 388 measurements: 159 in the summer and 229 in the winter. Newborn and infants aged from 2 days to 3 months were included. The time of collection of the exam was significant in the measurement of TSH and the mean levels of TSH collected in the summer were somewhat higher than those collected in the winter (p = 0,0001), when analyzed by the t-test, even so when the age was taken into account. Using F-test (p=0,0234) we observed differences between TSH levels in age groups. TSH levels in the newborn with 2 to 7 days of age differ significantly from those of higher age. Birth weight and prematurity do not show interference with the TSH measurement.Conclusions: Newborn and infant TSH levels were significantly higher in the summer than in the winter.