Usefulness of thyroxine and free thyroxine filter paper measurements in neonatal screening for congenital hypothyroidism of preterm babies

Introduction-Low thyroxine (T4) with normal thyroid stimulating hormone (TS H) is a well known condition in preterm (PT) infants. The establishment of T4 and freeT4 (FT4) values in filter paper dried blood spots in PT could pr ovide useful information in the neonatal period. Objective-To study T4 and FT4 levels in dried blood filter paper samples of PT and full term (FT) babies. Methods-We measured T4 by fluoroimmunoassay (FIA) DELFIA and TSH by IFMA DE LFIA (Wallac Inc Turku, Finland) in 193 PT (26 to 37 weeks of gestational a ge (GA)) in samples from the first and second week of life and in 153 FT ba bies in the first week of life. In 131 PT and 31 FT we determined FT4 in fi lter paper blood spots using FIA (Alonso Fernandez J). Infants were grouped according to GA. Results-There was a significant difference in T4 between PT and FT (p < 0.0 01). The lowest T4 levels were at 26 to 29 weeks GA. T4 values were lower i n the second week. FT4 in PT up to 35 weeks CA, during the first week, was significantly different with FT infants (G1 to G3 p < 0.01, G4 p < 0.05). F T4 values in the first and second weeks of life did not vary. Conclusions-T4 values were significantly lower in PT than in FT neonates, i ncreasing with GA. PT infants had low T4 with normal FT4 values. This could suggest a decreased thyroxine binding globulin (TBG) or decreased protein binding and/or an adaptative mechanism that would not require therapeutical intervention.