Comparative Study of Thyroxine Assays Employing Kit Radio-Ligand Reagents

Abstract
A comparative study of some ‘kit’ radiometric procedures for the determination of thyroxine in serum is reported. Four manufacturers' competitive-binding reagents and methods—three incorporating prior extraction and one employing column elution—are examined. Considerable discrepancies exist between the manufacturers' ‘standards’ which, in the main, account for the differences in results obtained by the different methods. Other deficiencies include susceptibility to metal ion interference, variation (up to 8%) in thyroxine extraction and non-specific, i.e. ‘mimic T-4’, measurement. Radio-ligand reagent ‘kits’ need to be critically assessed before being used routinely. Separation of hypo-and hyperthyroid from euthyroid patients is achieved as well with some competitive protein-binding T-4 methods as with serum PBI.