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HelpOrder Code LABFRT4D T4 (Thyroxine), Free, Dialysis, Serum
Additional Codes
Mayo Test Code: FRT4D
Test Down Notes
This test is temporarily unavailable. For additional details see test announcement here.
Reporting Name
T4 (Thyroxine), Free by Dialysis, SUseful For
Determining thyroid status of sick, hospitalized patients
Determining thyroid status of patients in whom abnormal binding proteins have been identified
Possibly useful in pediatric patients
Performing Laboratory
Mayo Clinic Laboratories in Rochester
Specimen Type
SerumOrdering Guidance
The routine free T4 is faster and provides useful information for most patients; order FRT4 / T4 (Thyroxine), Free, Serum.
Necessary Information
Include name and telephone number of contact physician
Specimen Required
Supplies: Sarstedt Aliquot Tube, 5 mL (T914)
Collection Container/Tube:
Preferred: Red top
Acceptable: Serum gel
Submission Container/Tube: Plastic vial
Specimen Volume: 2.6 mL
Collection Instructions:
1. Collect specimen immediately before next scheduled dose.
2. Within 2 hours of collection, centrifuge, and aliquot serum into a plastic vial.
Specimen Minimum Volume
1.2 mL
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| 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) Performed
Monday, Wednesday, Thursday
CPT Code Information
84439
Method Description
The tandem mass spectrometry method utilizes an equilibrium dialysis technique to separate free thyroxine (FT4) from serum proteins, including protein-bound T4. An isotope of thyroxine (L-thyroxine-[thyrosine-ring-(13)C6]) is added to the dialysate and serves as an internal standard. A liquid-liquid extraction with ethyl acetate further purifies the dialysate. After being dried down and reconstituted, the remaining extract is injected onto the tandem mass spectrometer where it undergoes high-throughput liquid chromatography combined with an electrospray ion source.(Unpublished Mayo method)
Reject Due To
| Gross hemolysis | Reject |
| Gross lipemia | Reject |
| Gross icterus | Reject |
Method Name
Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)