Order 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 |
---|---|---|
Serum | Refrigerated (preferred) | 7 days |
Frozen | 30 days | |
Ambient | 72 hours |
Reject Due To
Gross hemolysis | OK |
Gross lipemia | OK |
Gross icterus | OK |
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
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
Performing Laboratory

CPT Code Information
84479-Thyroxine binding capacity
84436-Thyroxine total
Method Name
TUPC: Electrochemiluminescence Immunoassay (ECLIA)
T4S: Electrochemiluminescence (ECL)
FRTI: Calculation