Thyroid function was evaluated in 75 healthy preterm infants, 30-35 weeks of gestational age. Serum thyrotropin (TSH), thyroxine (T(4)), triiodothyronine (T(3)), free T(4) (immunochemoluminescence) and reverse triiodothyronine (rT(3)) (radioimmunoassay) were measured in the mother and in the cord at delivery and in the preterm infants at 1 hour, 24 hours, 1 week, 3 weeks, 2 months, 4 months, 6 months, and 12 months of postnatal age. These values were compared to those of healthy full-term infants of the same postnatal age (22 at 24 hours from our hospital and from previously reported data at others times). Mean 24-hour TSH values were significantly lower (p < 0.001) in preterm than in full-term infant populations (12.38 +/- 6.13 microIU/mL versus 22.02 +/- 13.28 microIU/mL); however, all TSH values of preterm infants were in the range of the full-term values. Mean 24-hour free T(4) values were similar in preterm and full-term infants (1.88 +/- 0.46 ng/dL versus 2.01 +/- 0.54 ng/dL) and all preterm infants had free T(4) values within the range of those of full-term infants at 24 hours. Mean T(4) and T(3) values were significantly lower in preterm than in full-term neonates at 1 hour and 24 hours of age. Mean 24-hour rT(3) values were significantly higher in preterm than in full-term newborns. From 1 week onwards, all thyroid function values were in the same range in both populations. In conclusion, individual thyroid function was similar in healthy preterms and full-terms from the first 24 hours of life. Normative data in preterm infants during the first year of life applying the latest luminescence techniques currently used worldwide are reported.