Abstract
A simple radioimmunoassay has been developed for service purposes to determine serum total thyroxine levels. Only three additions are required, of standard or sample, labelled thyroxine and antibody in polyethylene glycol. After 2 hours' incubation at room temperature the antibody-bound and free fractions are separated by centrifugation. Serum total thyroxine levels were measured in 195 euthyroid subjects and it was established that normal values lay within the range 57 to 155 nmol/1. Serial blood samples taken over a 24-hour period, from 11 subjects, indicated that there was no circadian rhythm so that samples for total thyroxine assay can be taken at any time of the day. Similar results were obtained using serum or plasma. Satisfactory results were obtained for three quality control sera when measured by seven different laboratories using this method.