The determination of thyroxine levels in human plasma by double isotope-dilution technique

Abstract
A new method is described for the determination of thyroxine in the blood by double isotope-dilution technique using tritiated acetic anhydride as a radioactive reagent together with [131]-labelled free thyroxine or [14C]-labelled N-acetyl thyroxine as isotopic carriers. The method appears to estimate the readily available hormone in serum to an accuracy of 2–3%. The results of analysis of 32 samples of human serum taken from subjects having varying degrees of thyroid activity show that the thyroxine levels obtained follow the general pattern of thyroid activity very closely, in that increased thyroid function produces a high serum-thyroxine level. The ranges of values for hypofunction, euthyroid function, and hyperthyroid function were 0–3.5 μg/100 ml, 3.0–6.5 μg/100 ml, and greater than 6.5 μg/100 ml, respectively. The ratio of the thyroxine-iodine/protein-bound iodine showed a marked tendency to increase with rising thyroid function and the implications of this finding are briefly discussed.