Simultaneous determination of endogenous and 13C-labelled thyroid hormones in plasma by stable isotope dilution mass spectrometry

This study describes a capillary gas chromatography–mass spectrometry (GC–MS) method for the simultaneous determination of endogenous thyroid hormone (thyroxine, T4) and its 13C-labelled analogue (13C6-thyroxine) in plasma. 13C9-thyroxine was used as analytical internal standard. A double derivatization (CH3OH/HCl and HFBA) inducing good GC mobility was used for the GC–MS analysis of the thyroid hormones. Quantification was carried out by selected ion monitoring (SIM) of specific ions of the fragment ions (m/z 970/976/979). The detection limit of the present GC–MS–SIM method was found to be 100 pg per injection for thyroxine (S/N=3.0). A first implementation in in vivo tests of 13C6-T4 like metabolic tracer was carried out under veterinary control on one cat and one rabbit. The thyroxine follow-up was done by GC–MS and based on double isotopic dilution with two different regio-selective 13C-labelled molecules of the same hormone. The present paper discusses the possibilities and limitations of this methodology. The in vivo experiment demonstrated that the use of stable isotopes and mass spectrometry provide a reliable methodology for hormonal monitoring.