Free thyroxin measured in dried blood spots from normal, low-birth-weight, and hypothyroid neonates

We have adapted a new radioimmunoassay for free thyroxin (FT4) measurement in dried blood spots for use in neonatal screening for hypothyroidism. The method is easy, fast, and cheap. Within-assay and between-assay CVs are respectively 9.6% and 13.2%. In 997 neonates three days postpartum with normal thyrotropin concentrations, the mean FT4 concentration was 27.2 pmol/L (SD 7.3 pmol/L). There was no significant difference in mean FT4 concentration between boys and girls. FT4 concentrations increased linearly with birth weight or with gestational age, as expressed by multiple linear regression: FT4 (pmol/L) = 0.0016 birth weight (g) + 0.6931 gestational age (weeks) - 4.8772. Only gestational age significantly affected the FT4 value. For five hypothyroid infants tested on day three postpartum, FT4 values were all below the 1st percentile of values from healthy neonates. Thus, when the neonatal concentration of thyrotropin is above normal, FT4 measured in the same sample can provide a reliable earlier diagnosis of hypothyroidism.