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Test Code FT4 Dial T4 (Thyroxine), Free by Dialysis, Serum

Important Note

This test can only be ordered internally.  Please order a miscellaneous test stating "Free T4 by Equililbrium Dialysis."

Reporting Name

T4 (Thyroxine), Free by Dialysis, S

Useful For

Determining thyroid status of sick, hospitalized patients


Used where abnormal binding proteins are known to exist


Possibly useful in pediatric patients

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type


Specimen Required


Preferred: Red top

Acceptable: Serum gel

Specimen Volume: 2.6 mL

Collection Instructions: Serum must be separated from gel within 4 to 6 hours of draw.

Additional Information: Include name and telephone number of contact physician.

Specimen Minimum Volume

1.2 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Refrigerated (preferred) 28 days
  Frozen  21 days
  Ambient  7 days

Reference Values

0.8-2.0 ng/dL

Reference values apply to all ages.

Day(s) and Time(s) Performed

Monday, Wednesday, Thursday; 3 p.m.

Seattle Children's Hospital Note:

TAT: 5 - 10 days

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
FRT4D T4 (Thyroxine), Free by Dialysis, S 6892-4


Result ID Test Result Name Result LOINC Value
8859 T4 (Thyroxine), Free by Dialysis, S 6892-4

Method Description

The equilibrium dialysis method separates free thyroxine (FT4) from serum proteins and, thereby, also from protein-bound thyroxine (T4), before measuring it in the protein-free dialysate using sensitive, tandem mass spectrometry. The results are independent of the concentrations of the T4-binding proteins and unaffected by the presence of molecular variants of these proteins.(Soldin SJ, Soukhova N, Janicic N, et al: The measurement of free thyroxine by isotope dilution tandem mass spectrometry. Clin Chim Acta 2005 Aug;358:113-118)

Reject Due To


Mild OK; Gross reject


Mild OK; Gross reject


Mild OK; Gross OK



Method Name

Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)