Thyroid method 3b: Thyroxine-binding prealbumin (TTR) binding using fluorescence displacement (T4-FITC)

Method Description

This method investigates interference of test items with binding of thyroid hormones to human serum transport protein 'thyroxine-binding prealbumin (TTR)'. The principle of the method is that the addition of TTR-binding compounds to a mixture of TTR and a fluorescent conjugate FITC-T4 causes a decrease in fluorescence, because the TTR-binding compounds compete FITC-T4 out of the TTR binding-pocket and consequently lead to a decrease in the ratio of bound to unbound FITC-T4. The FITC-T4 conjugate exhibits higher fluorescence intensity when bound to TTR than in the unbound condition, probably due to the fact that the iodine atoms of the T4 present in the TTR binding pocket can no longer contribute to the intramolecular fluorescence quenching of the FITC-T4 conjugate. This method was selected with other methods to assess modulating effects on the thyroid system (OECD, 2014). Information on these selected methods is available in TSAR as ‘Thyroid methods’.