Measurement of 'Free Thyroxine' in Dried Blood Samples on Filter Paper by Radio- and Enzyme-Immunoassay for Monitoring Thyroid Function

Mass screening for congenital hypothyroidism in newborns has been developed because the irreversible mental retardation caused by this disorder can be prevented by early treatment. These programs include measurement of thyrotropin or total thyroxine (T4), or both, in dried blood samples. Measurement of total T4 is useful for detecting primary, secondary, and tertiary hypothyroidism, but gives false-positive results for subjects with abnormal concentrations of thyroxine-binding globulin (TBG), who do not need to be treated. Hence, we developed an enzyme immunoassay (EIA) and a radioimmunoassay (RIA) for measuring free T4 (FT4) in dried blood samples on filter paper.