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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 ​
Specimen Stability Information
Specimen Type Temperature T​​ime
Serum​ ​ ​ ​Refrigerated ​7 days
​Frozen ​30 days
​Ambient ​72 hours
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrument​​ation
Mayo Medical Laboratories Monday through Sunday ​​1 day ​Electrochemiluminescence Immunoassay
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 Comme​​nts
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 ​
Specimen Stability Information
Specimen Type Temperature T​​ime
Serum​ ​ ​ ​Refrigerated ​7 days
​Frozen ​30 days
​Ambient ​72 hours
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/Instrument​​ation
Mayo Medical Laboratories Monday through Sunday ​​1 day ​Electrochemiluminescence Immunoassay
For billing questions, see Contacts
Outreach CPTs
CPT Modifier
(if needed)		 Quantity Description Comme​​nts
84479 ​ ​1 ​Thyroxine binding capacity (TUP) ​
​84436 ​ ​1 ​Thyroxine Total ​
For most current information refer to the Marshfield Laboratory online reference manual.