Tyroksin, fritt (FT4, fritt T4) (hund)

Free T4 procedure is a direct or single test assay, in the sense that its results are not calculated as a function of total T4, but interpolated from a (stored) standard curve calibrated in terms of free T4 concentrations. In this respect it differs from classic equilibrium dialysis methods and from so-called free T4 index determinations as well. It requires neither a pre-incubation step nor preliminary isolation of the free fraction by dialysis or column chroma-tography. The assay employs several features to preserve the equilibrium between free and protein-bound T4 and to measure accurately the unbound fraction. First, optimal concentrations of blocking agents prevent ligand-labeled T4 analog from binding to endogenous proteins (including albumin) while maintaining native T4 binding characteristics. The blocking agents also minimize artifacts arising from abnormal levels of albumin or free fatty acids. Second, the T4 analog has a nondetectable binding affinity for TBG. Third, the assay’s specific antibody binds T4 about as tightly as albumin binds T4 and thereby avoids stripping the hormone from thyroid binding proteins. Finally, the procedure operates at physiological temperature, pH and ionic strength.(Metoden tatt i bruk: 8/3-04)