Comparison between free serum thyroxine levels, measured by analog and dialysis methods, in the presence of perfluorooctane sulfonate and perfluorooctanoate

Results from animal studies have shown that negative associations between serum levels of free thyroxine (FT4) and perfluorooctane sulfonate (PFOS) at concentrations much higher than those reported for any exposed population may be due to bias in analog methods used for measuring FT4. We aimed to assess if there is evidence of differences between human FT4 measurements in serum by an analog and a dialysis method due to the presence of PFOS or perfluorooctanoate (PFOA) in a population of 50 adults with typical US serum PFOS concentrations but higher PFOA concentrations. Mean analog–dialysis difference was −0.02 (95% CI = −0.06, 0.02). Regressing the difference between FT4 measurements on either PFOA or PFOS serum concentrations yielded slopes close to zero. The present findings do not indicate any observable bias from the use of the analog with respect to the dialysis method, across the range of PFOS and PFOA concentrations in this population.