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HelpTest Code LAB20765 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 |
Reject Due To
| Gross hemolysis | OK |
| Gross lipemia | OK |
| Gross icterus | OK |
Clinical Information
The determination of the total thyroxine (T4) concentration is of importance in laboratory diagnostics for differentiating between euthyroid, hyperthyroid, and hypothyroid conditions. As the major fraction of the total T4 is bound to transport proteins (thyroxine-binding globulin [TBG], prealbumin, and albumin), the determination of total T4 only provides correct information when the thyroxine-binding capacity (TBC) in serum is normal. The free thyroid hormones are in equilibrium with the hormones bound to the carrier proteins.
The TBC or T-uptake assay provides a measure of the available thyroxine-binding sites. Determination of the free thyroxine index (FTI) from the quotient of total T4 and thyroxine-binding index (ie, result of the T-uptake determination) takes into account changes in the thyroid hormone carrier proteins and the thyroxine level.
While total T4 is a relatively reliable indicator of T4 levels in the presence of normal binding proteins, it is not a reliable indicator when binding proteins are abnormal. For example, increases in thyroxine-binding proteins may cause increased total T4 levels despite normal free T4 levels and normal thyroid function.
Results are changed by drugs or physical conditions that alter the patient's TBG levels or drugs that compete with endogenous T4 and T3 for protein-binding sites.
Direct measurement of free thyroxine (FRT4 / T4 [Thyroxine], Free, Serum) has replaced the FTI test in most clinical situations.
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.
Method Description
Thyroxine Binding Capacity:
Thyroxine (T4) occupies the free binding sites in the serum sample. After addition of a T4-specific antibody labeled with a ruthenium complex, the T4-polyhapten and the antibody derivative react to form a complex, the concentration of which is inversely proportional to the concentration of the excess, exogenous T4. This immunological complex becomes bound to the added streptavidin-coated microparticles via interaction of biotin and streptavidin. Application of a voltage to the electrode then induces chemiluminescent emission which is measured by a photomultiplier.(Package insert: T-Uptake reagent. Roche Diagnostics; V3, 10/2022)
Total Thyroxine:
The Roche Elecsys T4 assay is a competitive assay using electrochemiluminescence detection. Bound T4 is released from binding proteins by 8-anilino-1-naphthalene sulfonic acid. Patient specimen is incubated with sheep polyclonal anti-T4 antibody labeled with ruthenium. Streptavidin-coated microparticles and biotinylated T4 are added for a second incubation during which the still free binding sites of the labeled antibody become occupied. The resulting immunocomplex becomes bound to the solid phase by interaction of biotin and streptavidin. The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are then removed and application of a voltage to the electrode induces the electrochemiluminescent emission. This signal is measured against a calibration curve to determine patient results.(Package insert: Elecsys T4. Roche Diagnostics; V 2.0, 01/2019)
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