THE RADIOIMMUNOASSAY OF THYROXINE IN UNEXTRACTED HUMAN SERUM

A specific, accurate, precise and simple radioimmunoassay for thyroxine (T4) in small volumes of unextracted serum is described. It discriminates well between levels found in different states of thyroid function. Levels in euthyroid subjects range between 4.3 and 11.2 μg/100 ml (mean 7.7); in untreated hyperthyroid subjects, 13.2‐25.6 μg/100 ml (mean 19.2); and in untreated hypothyroid subjects undetectable to 2.6 μg/100 ml (mean 1.4). There is satisfactory correlation with competitive protein binding and protein bound iodine methods. Because of its advantages over previous techniques, it is suggested that T4 radioimmunoassay will become the routine method of assessing thyroid status.The measurement of serum thyroxine (T4) is the single most important test in the routine laboratory assessment of thyroid status. Ideally the T4 assay should be specific, cheap, simple and have a high sample capacity. Thyroxine is determined most commonly in serum extracts by competitive protein binding (CPB) methods (Murphy & Pattee, 1964; Ekins et al., 1969) or, indirectly, by estimation of serum protein bound iodine (PBI). Both techniques require the processing of relatively large volumes of serum before assay. The PBI method has the added disadvantage of measuring iodine‐containing compounds other than thyroxine (Acland, 1971).We report here a specific, precise and simple radioimmunoassay for thyroxine in unextracted human serum and its application to the assessment of thyroid status. It has significant advantages over CPB, PBI and previous radioimmunoassay methods.