| Abstract: | 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. |
