Reliable free thyroxine (FT4) measurements are essential for assessing thyroid function and for correctly diagnosing and treating thyroid disorders. Thyroid hormones play an important role in normal brain development of the fetus, and abnormal FT4 during pregnancy is associated with adverse pregnancy outcomes. Standardization of FT4 measurements, is critical to improving the accuracy and reliability of current methods and thus to improve diagnosis, treatment and prevention of thyroidal illnesses. Currently, there are no serum-based reference materials available for FT4 to assess the accuracy and reliability of FT4 assays. CDC CSP is collaborating with the International Federation of Clinical Chemistry and Laboratory Medicine, and the Partnership for the Accurate Testing for Hormones to address these issues through development of an accurate and sensitive higher-order Reference Measurement Procedure (RMP) for FT4 that will be used to assign target value to serum-based materials. The CDC CSP FT4 reference method is using equilibrium dialysis in combination with liquid chromatography tandem mass spectrometry (LC-MS/MS). FT4 in serum is isolated from the binding proteins in 1 mL equilibrium dialysis cells for 4 hours at 37oC. FT4 is further isolated by extractions prior to LC-MS/MS analysis. To determine the concentration of FT4 in serum, certified primary reference materials are used to prepare calibration materials. Chromatographic separation is achieved using a C18 reverse phase column with a gradient of methanol and water with 0.1% formic acid. Quantification by selective reaction monitoring is performed in the positive mode using electrospray ionization. Two transitions are monitored for each analyte and internal standard, and triplicate injections are used to minimize instrument instability. The within- and between-day imprecision for the CDC RMP are 2.2–3.9% and 1.8–2.6%, respectively. By comparisons with the RMPs at Ghent University in Belgium, Radboud University Medical Center in the Netherlands, and the Reference Material Institute for Clinical Chemistry Standards in Japan, the CDC RMP reported a bias within +2.5% of the mean for all labs. Factors affecting measurement accuracy were investigated, to maximize recovery for optimum performance of the method. FT4 was detectable in all samples, and thus, suitable for analysis of hypo-, eu-, and hyperthyroid patients. A comparison among hypo-, eu-, and hyperthyroid patients of a commercially available immunoassay and the CDC RMP indicated a mean bias of -37.7%. The CDC CSP has also evaluated the RMP for accurate measurement of FT4 in pregnant individuals. This candidate reference method for FT4 in serum demonstrates good accuracy and precision, and can be used as a viable base for accuracy to which routine methods for FT4 can be compared. 1. Van Houcke, S.K., et. al. Clin. Chem. Lab. Med. 2011, 49, 1275–1281.