title: Isotope dilution-liquid chromatography/electrospray ionization-tandem mass spectrometry for the determination of serum thyroxine as a potential reference method.
abstract: A new candidate reference method is presented for the determination of thyroxine in serum. The method is based on isotope dilution-liquid chromatography/tandem mass spectrometry using electrospray for ionization. The internal standard used was 13C6-thyroxine, sample pretreatment consisted of protein precipitation and a two-step liquid/liquid extraction procedure, HPLC was performed on a C-18 column with an eluent containing methanol/water/formic acid (60:40:0.1, by volume), and finally thyroxine and its isotopically labeled analogue were measured in the selected reaction monitoring mode for the transitions m/z 777.7--> 731.7 and m/z 783.7--> 737.7, respectively. The detection limit for thyroxine was 6 pg, the within-run coefficient of variation was 1.1%. The samples were measured in six-fold: in duplicate on three independent days. The mean overall coefficient of variation of the method was 1.6%. The new method was evaluated by measuring nine control sera previously determined by an existing ID-GC/MS method. The differences between the results of the two methods ranged from-1.6% to +3.3%, with a mean of +0.2%.
authors.affiliation: Laboratoria voor Medische Biochemie en Klinische Analyse, Faculty of Pharmaceutical Sciences, University of Ghent, Belgium.
authors.forename: V I
authors.initials: VI
authors.lastname: De Brabandere
authors.forename: P
authors.initials: P
authors.lastname: Hou
authors.forename: D
authors.initials: D
authors.lastname: Stöckl
authors.forename: L M
authors.initials: LM
authors.lastname: Thienpont
authors.forename: A P
authors.initials: AP
authors.lastname: De Leenheer
mesh_terms: D002138:Calibration; D002851:Chromatography, High Pressure Liquid; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D013058:Mass Spectrometry; D012016:Reference Values; D015203:Reproducibility of Results; D012996:Solutions; D013974:Thyroxine / Q000097:blood*
chemical_list: D012996:Solutions; D013974:Thyroxine