Isotope dilution-gas chromatography/mass spectrometry (ID-GC/MS) and isotope dilution-liquid chromatography/tandem mass spectrometry (ID-LC/MS/MS) methods have been developed for an determination of triiodothyronine (T3) in serum and their potential as candidate reference methods investigated. In both methods, (13)C(9)-T3 was used as internal standard. Sample pretreatment consisted of deproteinization, extraction and high performance liquid chromatography (HPLC) purification. Conversion of serum thyroxine (T4) to T3 was controlled by adding (13)C(6)-T4. For GC/MS, T3 and (13)C(9)-T3 were converted to the N,O-di-heptafluorobutyryl (HFB) methyl ester derivatives and monitored at m/z 844 and 853. For LC/MS with electrospray ionization, the transitions m/z 652/661 to 606/614 were monitored. For use of the methods as candidate reference methods, special attention was paid to the calibration and the measurement protocol (duplicate analysis of each sample on three occasions). Evaluation of the ID-GC/MS and ID-LC/MS/MS methods showed the absence of interference by reverse T3 and T4, a limit of detection of 100 pg (GC/MS) and 18 pg (LC/MS), a recovery of 100 +/- 1.5% (95% confidence interval) and good precision (the total coefficient of variation, n = 6, was typically 1.5%). In addition to the recovery study, the accuracy of the methods was proven by method comparison (ID-GC/MS vs. LC/MS/MS) on three sera, showing a maximum deviation of 1.1%. Finally, the ID-GC/MS method was applied for measurement of 10 different human sera with a T3 concentration range from 0.6 to 7.3 ng/mL.