Spuriously high free thyroxine values in familial dysalbuminemic hyperthyroxinemia.

Cartwright et al. recently showed that current commercially available free thyroxine (FT4) 1 methods yield spuriously high results in individuals with familial dysalbuminemic hyperthyroxinemia (FDH) (1). In particular, variants of the “one-step labeled analog” technique have been notorious in this respect, whereas the 2-step approach, in which the sample is washed away before the antibody-bound T4 is brought into contact with the labeled T4 analog, is thought to eliminate the problem of tracer binding to albumin and especially albumin in FDH patients. Some 2-step assays, however, also yield spuriously high values in individuals with this condition; thus, interference is not simply a problem of 1-step assays. Inhibitors of T4 and T4 analog binding to the albumin included in the incubation buffer have been suggested to play a role for both the 1and 2-step designs. In addition, the report of Cartwright et al. referred to similarly increased results obtained by symmetric dialysis when the incubation buffer of the Beckman Coulter Access 2-step assay (known to give falsely increased results for FDH samples) was used. We present evidence for the most probable candidate substance that causes the spurious results, at least for this particular assay platform. The importance of the correct electrolyte composition of the medium used in assessing FT4 has been stressed repeatedly for the last