A direct free thyroxine (T4) immunoassay with the characteristics of a total T4 immunoassay.
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.
Fritz
Kristofer S
KS
Department of Biochemistry, Loma Linda University School of Medicine, Loma Linda, CA, USA.
Wilcox
R Bruce
RB
Nelson
Jerald C
JC
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
D013975:Thyroxine-Binding Proteins; D013974:Thyroxine