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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), S

Specimen Type

Serum


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 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

Mayo Clinic Laboratories in Rochester

CPT Code Information

84479-Thyroxine binding capacity

84436-Thyroxine total

Method Name

TUPC: Electrochemiluminescence Immunoassay (ECLIA)

T4S: Electrochemiluminescence (ECL)

FRTI: Calculation