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

Background: The 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 (I-125 or I-131) hormones. Methods: We 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 C-13(6)-labeled analogue in serum. The C-13(6)-thyroxine (C-13(6)-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. Results: Serum samples were prepared and concentrated by a solid-phase extraction chromatography method. The recovery rate was measured by I-125-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 (R-2) between radioimmunoassay (RIA) and isotope dilution-MALDI-TOF MS for detection of serum T4 was 0.982. Conclusion: This 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. (c) 2012 Elsevier B.V. All rights reserved.