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Direct measurement of FT3 by equilibrium dialysis, high-pressure liquid chromatography, and tandem mass spectrometry (ED-HPLC/MS-MS) provides enhanced accuracy over commonly used analog (automated) methods and is not affected by clinical conditions that impact protein binding capacity.1-4 Examples of clinical conditions in which the FT3 by dialysis and MS/MS method may be preferred include congenital absence of thyroxine-binding protein (TBG);1 pregnancy;1-5 administration of oral contraceptives or hormone therapy;6 administration of atypical antipsychotic medications;7 malignancy or other critical illnesses.8
1. Yue B, Rockwood Al, Sandrock T, La'ulu SL, Kushnir MM, Meikle AW. Free thyroid hormones in serum by equilibrium dialysis and online solid-phase extraction-liquid chromatography/tandem mass spectrometry. Clin Chem. 2008 Apr; 54(4):642-651. PubMed 18258669
2. Soldin SJ, Soukhova N, Janicic N, Jonklaas J, Soldin OP. The measurement of free thyroxine by isotope dilution tandem mass spectrometry. Clin Chim Acta. 2005 Aug; 358(1-2):113-118. PubMed 16018881
3. Nelson JC, Weiss RM, Wilcox RB. Underestimates of serum free thyroxine (T4) concentrations by free T4 immunoassays. J Clin Endocrinol Metab. 1994 Jul; 79(1):76-79. PubMed 8027258
4. Nelson JC, Tomei RT. Direct determination of free thyroxine In undiluted serum by equilibrium dialysis/radioimmunoassay. Clin Chem. 1988 Sep; 34(9):1737-1744. PubMed 3138040
5. Kahric-Janicic N, Soldin SJ, Soldin OP, West T, Gu J, Jonklaas J. Tandem mass spectrometry improves the accuracy of free thyroxine measurements during pregnancy. Thyroid. 2007 Apr; 17(4):303-311. PubMed 17465859
6. Mazer N. Interaction of estrogen therapy and thyroid hormone replacement in postmenopausal women. Thyroid. 2004; 14(Suppl 1):S27-34. PubMed 15142374
7. Surks MI, DeFesi CR. Normal serum free thyroid hormone concentrations in patients treated with phenytoin or carbamazepine. A paradox resolved. JAMA. 1996 May 15; 275(19):1495-1498. PubMed 8622224
8. De Groot L. Dangerous dogmas in medicine: The nonthyroidal illness syndrome. J Clin Endocrinol Metab. 1999 Jan; 84(1):151-164. PubMed 9920076
