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HelpTest Code FTI Free Thyroxine Index (FTI), Serum
Reporting Name
Free Thyroxine Index(FTI), SPerforming Laboratory
Mayo Clinic Laboratories in Rochester
Useful For
Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)
Specimen Required
Patient Preparation:
1. For 12 hours before specimen collection do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.
2. If patient is receiving treatment with lipid-lowering agents containing dextrothyroxine (D-T4), discontinue for 4 to 6 weeks prior to specimen collection.
Collection Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Submission Container/Tube: Plastic vial
Specimen Volume: 1 mL
Collection Instructions: Centrifuge and aliquot serum into a plastic vial.
Specimen Minimum Volume
0.5 mL
Specimen Type
SerumSpecimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 7 days | |
| Frozen | 30 days | ||
| Ambient | 72 hours |
Day(s) Performed
Monday through Sunday
Reference Values
THYROXINE BINDING CAPACITY (units are in Thyroxine Binding Index: TBI):
0-19 years: 0.8-1.2 TBI
≥20 years: 0.8-1.3 TBI
T4 TOTAL (T4):
0-5 days: 5.0-18.5 mcg/dL
6 days-2 months: 5.4-17.0 mcg/dL
3-11 months: 5.7-16.0 mcg/dL
1-5 years: 6.0-14.7 mcg/dL
6-10 years: 6.0-13.8 mcg/dL
11-19 years: 5.9-13.2 mcg/dL
≥20 years: 4.5-11.7 mcg/dL
FREE THYROXINE INDEX:
0-5 days: 5.1-20.8 mcg/dL
6 days-2 months: 5.5-18.0 mcg/dL
3-11 months: 5.7-16.8 mcg/dL
1-5 years: 5.9-15.0 mcg/dL
6-10 years: 6.0-13.9 mcg/dL
11-19 years: 5.9-13.2 mcg/dL
≥20 years: 4.8-12.7 mcg/dL
For International System of Units (SI) conversion for Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html
Profile Information
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| TUPC | Thyroxine Binding Capacity, S | No | Yes |
| T4S | Thyroxine, Total, S | Yes, (order T4) | Yes |
| FRTI | Free Thyroxine Index | No | Yes |
Method Name
TUPC: Electrochemiluminescence Immunoassay (ECLIA)
T4S: Electrochemiluminescence (ECL)
FRTI: Calculation
Test Classification
This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.CPT Code Information
84479-Thyroxine binding capacity
84436-Thyroxine total
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| FRTUP | Free Thyroxine Index(FTI), S | 32215-6 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| T4S | Thyroxine, Total, S | 83119-8 |
| TUPC | Thyroxine Binding Capacity, S | 74795-6 |
| FRTI | Free Thyroxine Index | 32215-6 |