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Test 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 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


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 SI unit Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Day(s) Performed

Monday through Sunday

Report Available

Same day/1 to 2 days

Performing 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