Sadler, W. A., Lynskey, Clare P., and Legge, M. (1978). Aust. Paediatr. J., 14, 154–182. Radioimmunoassay of thyroxine in dried blood spots. A rapid precise radioimmunoassay for thyroxine (T4) in dried blood spots on filter paper has been developed, and appears entirely suitable for use as a mass screening procedure for the early diagnosis of neonatal hypothyroidism.A systematic study of sampling factors which may influence levels of measured T4 suggests that non‐uniformity of sample preparation is the single greatest error source In a blood spot assay.The assay reaction mixture consists of a 4.5 mm diameter disc punched from the blood spot and 8‐anilino‐1‐naphthalene sulphonic acid, labelled T4 and T4 antiserum in 1.1 ml barbitone buffer. Following incubations for one hour at 37°C and overnight at 4°C, bound and free hormone are separated using dextran coated charcoal. Assay standards consist of a 2:3 mixture of a standardized serum and washed red cells.In 558 Infants screened to date T4 levels are not normally distributed, being skewed towards the lower end, with a median value of 179 nmol/l. One Infant with congenital hypothyroidism has been found.