Problems in screening for congenital hypothyroidism using thyroxine assays from small dried blood discs

The feasibility of screening for neonatal hypothyroidism by initial thyroxine (T4) measurements from small (3.2 mm diam.) discs of dried heel or cord blood was assessed using double-antibody radioimmunoassay and solid-phase radioassay kits. Adult heparinized venous blood or finger-prick capillary blood served as references. Reproducibility of results was satisfactory in adult but not infant samples, probably because of problems in collecting and handling the latter. In comparative studies, most of the 16 infants with congenital primary hypothyroidism had within-run T4 values above the 2.5 percentile; some were as high as the 7th and the 5th percentile for heel and cord blood, respectively. Simultaneously determined heel and cord dried-blood thyrotropin (TSH) values were clearly increased to greater than 40 microU/ml in 15 of these infants. Thus, when small discs are used for initial blood T4 screening to detect primary hypothyroidism, a higher cut-off points is recommended to safeguard against the observed technical problems.