Isotope dilution tandem mass spectrometric method for T4/T3

The thyroid hormones thyroxine (T4) and 3,5,3'-triidothyronine (T3) are essential for regulating a number of biological processes, including growth, neurodevelopment, carbohydrate metabolism, oxygen consumption and protein synthesis. Immunoassays are the current methods for thyroid hormone measurement and suffer from a lack of specificity. Our objective was to simultaneously measure T4 and T3 using isotope dilution tandem mass spectrometry within a single run. To compare the results obtained by this MS/MS method with those obtained by an immunoassay procedure on the same samples (DPC Immulite for T3, Diagnostics Product, and Dade RxL Dimension for T4, Dade-Behring).

An API-3000 tandem mass spectrometer (SCIEX, Toronto, Canada) equipped with TurboIonSpray and Shimadzu HPLC system was used employing isotope dilution with deuterium-labeled internal standard (l-thyroxin-d2). The method requires 100 microl of serum and involves addition of internal standard, precipitation of proteins with methanol and injection of the supernatant onto a C-18 column. After washing, the switch valve is activated and T4 and T3 eluted using a methanol gradient. T4 and T3 by immunoassay were performed using the Dade RxL Dimension and the DPC Immulite, respectively.

An isotope dilution tandem mass spectrometry method for the simultaneous determination of total T4 and T3 in serum is described which is accurate, specific, precise (%CVs 3.5-9.0), simple and fast (<7 min).