title: A direct free thyroxine (T4) immunoassay with the characteristics of a total T4 immunoassay.
abstract: BACKGROUND
  Direct free thyroxine (T(4)) measurements have been linked to both T(4)-binding serum protein concentrations and protein-bound T(4) concentrations. Whether this is evidence of a relationship to total T(4) concentrations has not been reported.
  
  
  METHODS
  We compared an analog-based direct free T(4) immunoassay and a total T(4) immunoassay. Each assay was applied to the fractions of serum T(4) obtained by ultrafiltration and equilibrium dialysis. Both were applied to serum-based solutions in which free T(4), T(4)-binding proteins, protein-bound T(4), and total T(4) were systematically varied, held constant, or excluded.
  
  
  RESULTS
  Neither the free T(4) assay nor the total T(4) assay detected dialyzable or ultrafilterable serum T(4). Both assays detected and reported the T(4) retained with serum proteins. Both free and total T(4) results were related to the same total T(4) concentrations in the presence and absence of T(4)-binding proteins. Both results were similarly related to total T(4) concentrations when free T(4) was held constant while total T(4) was varied. Both were similarly related to a total T(4) concentration that was held constant while free T(4) progressively replaced protein-bound T(4). These free T(4) results, like total T(4) results, were unresponsive to a 500-fold variation in dialyzable T(4) concentrations.
  
  
  CONCLUSIONS
  New experiments extend the characterization of a longstanding and incompletely characterized analog-based free T(4) immunoassay. These free T(4) measurements relate to total T(4) concentrations in the same way that total T(4) measurements do.
authors.affiliation: Department of Biochemistry, Loma Linda University School of Medicine, Loma Linda, CA, USA.
authors.forename: Kristofer S
authors.initials: KS
authors.lastname: Fritz
authors.forename: R Bruce
authors.initials: RB
authors.lastname: Wilcox
authors.forename: Jerald C
authors.initials: JC
authors.lastname: Nelson
mesh_terms: D000328:Adult; D001774:Blood Chemical Analysis; D001800:Blood Specimen Collection / Q000379:methods; D003956:Dialysis; D006801:Humans; D007118:Immunoassay / Q000379:methods; D008297:Male; D008875:Middle Aged; D011485:Protein Binding; D013974:Thyroxine / Q000097:blood* / Q000378:metabolism; D013975:Thyroxine-Binding Proteins / Q000378:metabolism*; D014462:Ultrafiltration
chemical_list: D013975:Thyroxine-Binding Proteins; D013974:Thyroxine