Free thyroxine values in dried blood spots on filter paper in newborns are related to both gestational age and birth body weight

The results of free thyroxine (FT4) measurements in dried blood spots on filter paper in 744 euthyroid newborns (616 at term, 128 preterm), 10 newborns with congenital hypothyroidism and 4 euthyroid newborns with congenital TBG deficiency are reported. FT4 was measured by column adsorption chromatography of free hormone followed by radioimmunoassay in the eluate. FT4 values averaged 24 ± 0.2 pmol/L (mean ± SE) in euthyroid newborns, 23.0 ± 0.9 pmol/L in euthyroid newborns with TBG deficiency (p = NS), and 5.7 ± 0.4 pmol/L in hypothyroid newborns (p <0.001 vs both groups). Total T4 (TT4) values in newborns with TBG deficiency were not different from those in hypothyroid newborns, but were significantly lower than those in euthyroid newborns without TBG abnormalities. FT4 values were higher in full-term newborns than in preterm newborns (25.2 ± 0.3 vs 21.2 ± 0.5 pmol/L, p 2,500 g, respectively) showed higher specificity and predictive value of positive results than the cut-off values based on the gestational age. In any case, the sensitivity, specificity and predictive values of FT4 determinations proved to be higher than those of TT4 and TSH measurements. In conclusion: i) FT4 can be measured in dried blood spots on filter paper, advantageously replacing TT4 determination; ii) the assessment of thyroid status in newborns based on FT4 measurement must take into account the gestational age or, more reliably, the birth body weight.