Stable isotope-dilution liquid chromatography/tandem mass spectrometry method for determination of thyroxine in saliva

A liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) method for the determination of thyroxine (T4) in human saliva has been developed and validated. The saliva was deproteinized with methanol, purified using a Strata-X™ cartridge, and subjected to LC/ESI-MS/MS. Quantification was based on selected reaction monitoring, and [13C6]-T4 was used as the internal standard. This method allowed the reproducible (intra- and inter-assay relative standard deviations, <4.8%) and accurate (analytical recovery, 96.5–99.6%) quantification of the salivary T4 using a 400 μl sample, and the limit of quantification was 25.0 pg/ml. A preliminary study using the developed method found that there is a diagnosable difference in the salivary T4 concentration between the euthyroid subjects and the patients with Graves disease.