Free thyroxin measured in dried blood spots: results for 10 000 euthyroid and 29 hypothyroid newborns

We measured free thyroxin (FT4) in dried blood samples from 10 000 euthyroid and 29 hypothyroid newborns (three with transient hypothyroidism and 26 permanent). In euthyroids, the mean FT4 concentration was 16.9 (SD 4.5) pmol/L. Preterm babies had low concentrations of FT4, and values were closely related to birth weight when less than 2500 g. Assay of blood-spot FT4 appears to be specific as a test for hypothyroidism screening, yielding 0.42% false positives when a mean minus 2 SD cutoff value (8 pmol/L) was used. Correcting FT4 for birth weight further decreased the false-positive rate, to 0.07%. In all three newborns with transient hypothyroidism, FT4 was less than 8 pmol/L. In 25 of 26 permanent hypothyroids, FT4 ranged from undetectable to 5 pmol/L; in the 26th infant, who had a large ectopic gland, it was 8.2 pmol/L. We believe that FT4 assay offers an attractive improvement over total T4, whether performed as the first screening test or as a confirmatory test in thyrotropin screening programs.