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25510 Free Thyroxine Index (FTI), S (FRTUP)

Free Thyroxine Index (FTI), S (FRTUP)
Test Code: FRTUPSO
Synonyms/Keywords
Thyroxine Binding Capacity, TBC, T-Uptake, T4-Uptake, Thyroxine Binding Index, TBI
Test Components
Thyroxine Binding Capacity, Thyroxine Total, Free Thyroxine Index
Useful For
​​Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)
Specimen Requirements
Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Serum ​Serum Separator Tube (SST) ​Red Top Tube (RTT) ​1.0 mL ​0.5 mL
Collection Processing Instructions

​For 12 hours before specimen collection do not take multivitamins or dietary supplements conaining biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

If patient is receiving treatment with lipid-lowering agents containing D-T4, discontinue for 4 to 6 weeks prior to specimen collection.

Specimen Stability Information
Specimen Type Temperature Time
Serum​ ​ ​ ​Refrigerated (preferred) ​7 days
​Frozen ​30 days
​Ambient ​72 hours
Interference

​This test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established.

Autoantibodies to thyroid hormones can interfere with the assay.

Binding protein anomalies seen with familial dysalbuminemic hyperthyroxinemia, for example, may cause values which, while characteristic of the condition, deviate from the expected results.

Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
Mayo Clinic Laboratories Monday through Sunday ​​1-2 days ​Electrochemiluminescence Immunoassay
Test 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 by immunoassay) has replaced the FTI test in most clinical situations.

Reference Range Information

Thyroxine Binding Capacity (units are in Thyroxine Binding Index: TBI):
0-19 years: 0.8-1.2 TBI
> or =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
> or =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
> or =20 years: 4.8-12.7 mcg/dL

Interpretation

The Free Thyroxine index (FTI) is determined by the following calculation: FTI = 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.

Outreach CPTs
CPT Modifier
(if needed)
Quantity Description Comments
84479 ​1 ​Thyroxine binding capacity (TUP)
​84436 ​1 ​Thyroxine Total
Synonyms/Keywords
Thyroxine Binding Capacity, TBC, T-Uptake, T4-Uptake, Thyroxine Binding Index, TBI
Test Components
Thyroxine Binding Capacity, Thyroxine Total, Free Thyroxine Index
Ordering Applications
Ordering Application Description
​COM ​Free Thyroxine Index (FTI)
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)
Pediatric Minimum Volume
(no repeat)
​Serum ​Serum Separator Tube (SST) ​Red Top Tube (RTT) ​1.0 mL ​0.5 mL
Collection Processing

​For 12 hours before specimen collection do not take multivitamins or dietary supplements conaining biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

If patient is receiving treatment with lipid-lowering agents containing D-T4, discontinue for 4 to 6 weeks prior to specimen collection.

Specimen Stability Information
Specimen Type Temperature Time
Serum​ ​ ​ ​Refrigerated (preferred) ​7 days
​Frozen ​30 days
​Ambient ​72 hours
Interference

​This test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established.

Autoantibodies to thyroid hormones can interfere with the assay.

Binding protein anomalies seen with familial dysalbuminemic hyperthyroxinemia, for example, may cause values which, while characteristic of the condition, deviate from the expected results.

Useful For
​​Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)
Test Components
Thyroxine Binding Capacity, Thyroxine Total, Free Thyroxine Index
Reference Range Information

Thyroxine Binding Capacity (units are in Thyroxine Binding Index: TBI):
0-19 years: 0.8-1.2 TBI
> or =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
> or =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
> or =20 years: 4.8-12.7 mcg/dL

Interpretation

The Free Thyroxine index (FTI) is determined by the following calculation: FTI = 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.

For more information visit:
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
Mayo Clinic Laboratories Monday through Sunday ​​1-2 days ​Electrochemiluminescence Immunoassay
For billing questions, see Contacts
Outreach CPTs
CPT Modifier
(if needed)
Quantity Description Comments
84479 ​1 ​Thyroxine binding capacity (TUP)
​84436 ​1 ​Thyroxine Total
For most current information refer to the Marshfield Laboratory online reference manual.