Triiodothyronine (T3), Free, Dialysis and LC/MS-MS (Endocrine Sciences)

CPT: 84481
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  • Free T3
  • Free Triiodothyronine (T3)
  • FT3

Expected Turnaround Time

6 - 10 days

Related Documents

Specimen Requirements


Serum (preferred) or plasma, frozen


1 mL

Minimum Volume

0.3 mL (Note: This volume does not allow for repeat testing).


Red-top tube, gel-barrier tube, lavender-top (EDTA) tube, or green-top (sodium heparin) tube


Serum/plasma must be separated from cells within 45 minutes of venipuncture. Send serum/plasma in a plastic transfer tube.

Storage Instructions

Freeze (preferred) or refrigerate. Stable at room temperature or refrigerated for five days. Stable frozen for 70 days.

Causes for Rejection

Whole blood

Test Details


Equilibrium dialysis and high-pressure liquid chromatography/tandem mass spectrometry (HPLC/MS-MS)

Reference Interval

See table.


Range (pg/mL)

Mean (pg/mL)

Premature (0 to 4 wk)




0 to 2 wk



2 wk to 12 m



Prepubertal 1 to 10 y



Children >10 y and adults



Pregnant female

first trimester



second trimester

To be determined

third trimester



Additional Information

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. 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. 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. 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. 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. 17465859
6. Mazer N. Interaction of estrogen therapy and thyroid hormone replacement in postmenopausal women. Thyroid. 2004; 14(Suppl 1):S27-34. 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. 8622224
8. De Groot L. Dangerous dogmas in medicine: The nonthyroidal illness syndrome. J Clin Endocrinol Metab. 1999 Jan; 84(1):151-164. 9920076


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
503600 T3, Free, Dialysis, LC/MS-MS 503601 T3, Free, Dialysis, LC/MS-MS pg/mL 29239-1

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