ExtractA rapid, sensititive radioimmunoassay for thyroxine (T4) is described which requires a specimen of dried blood on filter paper. One milliliter of glycine-acetate buffer containing anti-T4antibody, tracer T4, and sodium salicylate is added to a tube containing a 1/8-inch dot of the filter paper specimen. After, incubation overnight, bound and free hormone are separated by addition of dextran-coated charcoal. Quantitation is obtained using a standard curve prepared from dots of dried blood samples with known T4content. The dot remains in the solution throughout the procedure. Recovery of T4is 95% and intra- and interassay coefficients of variation are both less than 10%. The mean T4content of 983 samples from 3-day-old infants was 189 ± 48 pg T4/dot (mean SD). This corresponds to the T4 in 1.5 μl plasma, and thus the estimated plasma T4in these infants is 12.6 ± 3.2 μg T4/100 ml. Nine neonates had repeated samples in which the T4content was lower than 2 SD below the mean. All of these infants had normal cord thyroid-stimulating hormone (TSH) concentrations and thus presumably do not have primary hypothyroidism. The method …