Reference methods for the measurement of free thyroid hormones in blood: evaluation of potential reference methods for free thyroxine.

In this paper, a review of the literature on the methods for the detection of FT(4) is provided. Furthermore, an overview of the most important uncertainties and interferences of the potential reference methods for determination of free thyroxine (free triiodothyronine) are also presented. Especially, the separation step-dialysis, or ultrafiltration-implies important technical and theoretical uncertainties and interferences as to the methods.Furthermore, a short review of methods based on isotope dilution/mass spectrometry (ID/MS) to quantify thyroxine is provided. There has so far been no attempt to quantify the free fraction of thyroxine by ID/MS, probably due to lack of sensitivity, but if quantification of the free fraction of thyroxine can be done with ID/MS, then this step seems the least compromised step only including minor uncertainties in comparison to the separation step.