Utilizing isotope dilution-matrix-assisted laser desorption ionization-time of flight mass spectrometry as a reference procedure for the radioimmunoassay of serum thyroxine

BackgroundThe thyroid hormone, thyroxine (T4), is a tyrosine-based hormone produced by the thyroid gland, which is essential in regulating a number of biological processes, including growth, neurodevelopment, carbohydrate metabolism, oxygen consumption and protein synthesis. Data on human thyroid hormone metabolism were gathered since the middle of the 1970s mainly by the use of radioactive iodinated (125I or 131I) hormones.MethodsWe describe an isotope dilution-matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) method for the simultaneous determination of endogenous thyroid hormone and its 13C6-labeled analogue in serum. The 13C6-thyroxine (13C6-T4) was used as an internal standard; T4 and its isotopically labeled analogue were measured in the selected reaction monitoring mode for the transitions from m/z 777.8 to 732.1 and from m/z 784.2 to 738.1, respectively.ResultsSerum samples were prepared and concentrated by a solid-phase extraction chromatography method. The recovery rate was measured by 125I-T4 and can be up to 82.8 ± 2.8%. The detection limit and linear range for T4 were 5 ng/ml and 5–400 ng/ml, respectively. The correlation coefficient (R2) between radioimmunoassay (RIA) and isotope dilution-MALDI-TOF MS for detection of serum T4 was 0.982.ConclusionThis assay has a good relationship against a commercial RIA and the isotope dilution-mass spectrometry method and may serve as a reference method for quantitative analysis of T4.

► This study describes a MALDI-TOF MS method for the determination of T4 in serum.
► The technique is simple and easy to apply on MALDI-TOF MS for T4 analysis.
► The correlation coefficient between MALDI-TOF MS and RIA was 0.982 for T4 in sera.
► Isotope dilution-MALDI-TOF MS may develop a reference method for T4 analysis.