title: Tandem mass spectrometry improves the accuracy of free thyroxine measurements during pregnancy.
abstract: OBJECTIVE
  Pregnancy is a time of rapidly changing demands on the thyroid axis, and knowledge of thyroid hormone levels, especially during the first trimester, is important for ensuring maternal and fetal health. The thyroid hormone assays currently in use become more inaccurate at extremes of binding protein concentrations and when heterophilic antibodies are present. Pregnancy is characterized by both these conditions, making accurate determination of free thyroid hormone levels by conventional direct analog immunoassay methods difficult. The objective of this study was to characterize the performance of a novel tandem mass spectrometric assay for free thyroxine during the physiologic conditions of pregnancy.
  
  
  METHODS
  Healthy women without a history of thyroid abnormalities were recruited from the obstetrics and gynecology and endocrinology clinics of a university medical center and their thyroid status was monitored. Free thyroxine levels were assessed by both immunoassay and tandem mass spectrometry during the course of their pregnancy. Serum thyrotropin levels were also measured. The distributions of free thyroid concentrations obtained by the two assays were compared.
  
  
  RESULTS
  The tandem mass spectrometry and immunoassay values did not correlate well with each other. However, tandem mass spectrometry values correlated well with the current gold standard equilibrium dialysis values. Moreover, the good agreement between equilibrium dialysis and tandem mass spectrometry was maintained across all weeks of gestation.
  
  
  CONCLUSIONS
  We conclude that tandem mass spectrometry has a superior performance to immunoassay for the measurement of free thyroxine during pregnancy. Furthermore, it is ideally suited to generating trimester-specific reference intervals for free thyroxine levels. Future studies will determine if it is a better assay to use in most clinical circumstances.
authors.affiliation: Division of Endocrinology, Georgetown University Medical Center, Washington, DC, USA.
authors.forename: Natasa
authors.initials: N
authors.lastname: Kahric-Janicic
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Soldin
authors.forename: Offie P
authors.initials: OP
authors.lastname: Soldin
authors.forename: Threvia
authors.initials: T
authors.lastname: West
authors.forename: Jianghong
authors.initials: J
authors.lastname: Gu
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Jonklaas
mesh_terms: D000328:Adult; D002853:Chromatography, Liquid; D003956:Dialysis; D005260:Female; D006801:Humans; D007118:Immunoassay; D011247:Pregnancy / Q000097:blood*; D011261:Pregnancy Trimester, First / Q000097:blood; D053719:Tandem Mass Spectrometry / Q000379:methods*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*
chemical_list: D013972:Thyrotropin; D013974:Thyroxine