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HelpTest Code FT4 Dial T4 (Thyroxine), Free, Dialysis, Serum
Test Down Notes
Effective August 23, 2024: This test is temporarily unavailable due to increased volume of testing requests. The downtime is expected to be >30 days. Order FFT4T as an alternative. See test notification 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
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. It has not been cleared or approved by the US Food and Drug Administration.CPT Code Information
84439
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 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)
Report Available
3 to 8 daysReject Due To
| Gross hemolysis | Reject |
| Gross lipemia | Reject |
| Gross icterus | Reject |
Method Name
Equilibrium Dialysis/Tandem Mass Spectrometry (MS/MS)