basis because of the possibility of variations in instrumental conditions as shown in an earlier study (21). For this purpose, multiple ion detection using an accelerating voltage alternator (AVA) (23) and simultaneous monitoring of several ions, including one or more of the isotopically labeled internal standard, is necessary. Ideally, the internal standard should be subjected to the identical sample preparation and derivatization conditions as the compound in question. This was not possible in this specific case, as the labeled analogue of RT3 had to be prepared separately during esterification. This problem can be alleviated by using a compound containing deuterium atoms on the thyronine phenyl rings, which, of course, would be a major project in itself. Nevertheless, the methodology developed for the analysis of free RTB by Dialysis-GC-MS-SIM can be applied to the analysis of all the thyroid hormones and their precursors as their HFB-ME derivatives.