Thyroid Function in Seventy-Five Healthy Preterm Infants Thirty to Thirty-Five Weeks of Gestational Age: A Prospective and Longitudinal Study During the First Year of Life

Abstract

Thyroid function was evaluated in 75 healthy preterm infants, 30-35 weeks of gestational age. Serum thyrotropin (TSH), thyroxine (T₄), triiodothyronine (T₃), free T₄ (immunochemoluminescence) and reverse triiodothyronine (rT₃) (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 μIU/mL versus 22.02 ± 13.28 μIU/mL); however, all TSH values of preterm infants were in the range of the full-term values. Mean 24-hour free T₄ 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 T₄ values within the range of those of full-term infants at 24 hours. Mean T₄ and T₃ values were significantly lower in preterm than in full-term neonates at 1 hour and 24 hours of age. Mean 24-hour rT₃ 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.