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Test ID 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


Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Specimen Volume: 1 mL

Additional Information: 12 hours before this blood test do not take multivitamins or dietary supplements containing biotin or vitamin B7, which are commonly found in hair, skin, and nail supplements and multivitamins.


Specimen Minimum Volume

0.5 mL

Specimen Stability Information

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

Clinical 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 thyroxine 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 in serum is normal. The free thyroid hormones are in equilibrium with the hormones bound to the carrier proteins.

 

The thyroid binding capacity 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 (TBI) (TBI=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 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

Interpretation

The free thyroxine index (FTI) is determined by the following calculation:

FTI = Thyroxine (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.

Clinical Reference

1. Whitley RJ, Meikle AW, Watts NB: Thyroid function. In Tietz Fundamentals of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 1996, pp 645-646

2. Wilson JD, Foster DW, Kronenburg MD, et al: Williams Textbook of Endocrinology. Ninth edition, WB Saunders Company, 1998, pp 407-477

Day(s) and Time(s) Performed

Monday through Sunday; Continuously

Analytic Time

Same day/1 day

Test Classification

This test has been cleared or approved by the U.S. 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(TUP)

84436-Thyroxine total (T4S)

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 3026-2
TUPC Thyroxine Binding Capacity, S 74795-6
FRTI Free Thyroxine Index 32215-6

Method Name

Electrochemiluminescence Immunoassay