title: Total and free thyroxine and triiodothyronine: measurement discrepancies, particularly in inpatients.
abstract: OBJECTIVE
  We compared the performance of tandem mass spectrometry versus immunoassay for measuring thyroid hormones in a diverse group of inpatients and outpatients.
  
  
  METHODS
  Thyroxine (T4), triiodothyronine (T3), free thyroxine (FT4), and free triiodothyronine (FT3) were measured by liquid chromatography tandem mass spectrometry and immunoassay in 100 patients and the two assays were compared.
  
  
  RESULTS
  T4 and T3 values measured by the two different assays correlated well with each other (r=0.91-0.95). However, the correlation was less good at the extremes (r=0.51-0.75). FT4 and FT3 concentrations measured by the two assays correlated less well with each other (r=0.75 and 0.50 respectively). The studied analytes had poor inverse correlation with the log-transformed TSH values (r=-0.22-0.51) in the population as a whole. The strongest correlations were seen in the groups of outpatients (r=-0.25-0.61). The weakest degree of correlation was noted in the inpatient group, with many correlations actually being positive.
  
  
  CONCLUSIONS
  The worst between-assay correlation was demonstrated at low and high hormone concentrations, in the very concentration ranges where accurate assay performance is typically most clinically important. Based on the lesser susceptibility of mass spectrometry to interferences from conditions such as binding protein abnormalities, we speculate that mass spectrometry better reflects the clinical situation. In this mixed population of inpatients and outpatients, we also note failure of assays to conform to the anticipated inverse linear relationship between thyroid hormones and log-transformed TSH.
authors.affiliation: Division of Endocrinology, Georgetown University, Washington, DC, USA. Electronic address: jonklaaj@georgetown.edu.
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Jonklaas
authors.affiliation: Division of Endocrinology, Georgetown University, Washington, DC, USA; Section of Endocrinology Medstar Washington Hospital Center, Washington, DC, USA.
authors.forename: Anpalakan
authors.initials: A
authors.lastname: Sathasivam
authors.affiliation: Medstar Health Research Institute, Hyattsville, MD, USA.
authors.forename: Hong
authors.initials: H
authors.lastname: Wang
authors.affiliation: Department of Laboratory Medicine, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Jianghong
authors.initials: J
authors.lastname: Gu
authors.affiliation: Section of Endocrinology Medstar Washington Hospital Center, Washington, DC, USA.
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Burman
authors.affiliation: Department of Laboratory Medicine, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Soldin
mesh_terms: D002853:Chromatography, Liquid / Q000379:methods; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000379:methods; D007297:Inpatients; D008297:Male; D008875:Middle Aged; D010045:Outpatients; D053719:Tandem Mass Spectrometry / Q000379:methods; D013960:Thyroid Function Tests / Q000379:methods; D013963:Thyroid Hormones / Q000097:blood; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D013963:Thyroid Hormones; D014284:Triiodothyronine; D013974:Thyroxine
keywords: Free thyroxine; Free triiodothyronine; Immunoassays; Tandem mass spectrometry; Thyroxine; Triiodothyronine