Isotope dilution--mass spectrometry of thyroxin proposed as a reference method

We describe a highly accurate and specific method based on isotope dilution--mass spectrometry for assay of thyroxin in serum. A fixed amount of 2H2-labeled thyroxin is added to a fixed amount of serum. Thyroxin is isolated from the serum by an extraction--solvent-distribution method, converted into the N,O-bis(trifluoroacetyl)methyl ester, and subjected to combined gas chromatography--mass spectrometry. The amount of thyroxin is determined from recordings at m/e 799 and m/e 801. These two ions correspond to the M-184 peak in the mass spectrum of the derivative of unlabeled and 2H2-labeled thyroxin, respectively. With this derivative and with the specific gas-chromatographic conditions used, the "memory" effects observed in previous work were absent. The accuracy of the method was checked by analyses of serum samples without addition of internal standard and by different recovery experiments. The intra-assay variation (CV) was less than 2%, the interassay variation (CV) less than 3.5%. We suggest that this method may be used as a Reference Method. Results by the present method were compared with those by a commercial radioimmunoassay. To our knowledge this represents the first time that radioimmunoassay of thyroxin has been evaluated with a more nearly accurate method. There was excellent agreement between results by the two methods (r = 0.997). We conclude that radioimmunoassay of serum thyroxin is sufficiently accurate for use in clinical work.