Home
HelpTest Code FRTUP Free Thyroxine Index (FTI), Serum
Useful For
Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)
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 |
Reporting Name
Free Thyroxine Index(FTI), SSpecimen Type
SerumSpecimen 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 Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 7 days | |
| Frozen | 30 days | ||
| Ambient | 72 hours |
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
Interpretation
The free thyroxine index (FTI) is determined by the following calculation:
FTI =thyroxine (T4)/thyroid binding capacity
The FTI is a normalized determination that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins.
Hyperthyroidism causes increased FTI, and hypothyroidism causes decreased values.
Clinical Reference
1. Whitley RJ, Meikle AW, Watts NB. Thyroid function. In: Burtis CA, Ashwood, ER, eds. Tietz Fundamentals of Clinical Chemistry. 4th ed. WB Saunders Company; 1996:645-646
2. Wilson JD, Foster DW, Kronenburg MD, et al. Williams Textbook of Endocrinology. 9th ed. WB Saunders Company; 1998:407-477
3. Freedman DB, Halsall D, Marshall WJ, Ellervik C. Thyroid disorders. In: Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1572-1616
4. Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343-1421
5. Persani L, Cangiano B, Bonomi M. The diagnosis and management of central hypothyroidism in 2018. Endocr Connect. 2019;8(2):R44-R54. doi: 10.1530/EC-18-0515
Day(s) Performed
Monday through Sunday
Report Available
Same day/1 to 2 daysPerforming Laboratory
Mayo Clinic Laboratories in Rochester
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 |
Method Name
TUPC: Electrochemiluminescence Immunoassay (ECLIA)
T4S: Electrochemiluminescence (ECL)
FRTI: Calculation