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Test Id : FRTUP

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Free Thyroxine Index (FTI), Serum

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Useful For
Suggests clinical disorders or settings where the test may be helpful

Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)

Profile Information
A profile is a group of laboratory tests that are ordered and performed together under a single Mayo Test ID. Profile information lists the test performed, inclusive of the test fee, when a profile is ordered and includes reporting names and individual availability.

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

Method Name
A short description of the method used to perform the test

TUPC: Electrochemiluminescence Immunoassay (ECLIA)

T4S: Electrochemiluminescence (ECL)

FRTI: Calculation

NY State Available
Indicates the status of NY State approval and if the test is orderable for NY State clients.

Yes

Reporting Name
Lists a shorter or abbreviated version of the Published Name for a test

Free Thyroxine Index(FTI), S

Aliases
Lists additional common names for a test, as an aid in searching

T-Uptake

T4-Uptake

Thyroxine Binding Capacity-TBC

Thyroxine Binding Index-TBI

Specimen Type
Describes the specimen type validated for testing

Serum

Specimen Required
Defines the optimal specimen required to perform the test and the preferred volume to complete testing

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
Defines the amount of sample necessary to provide a clinically relevant result as determined by the Testing Laboratory

0.5 mL

Reject Due To
Identifies specimen types and conditions that may cause the specimen to be rejected

Gross hemolysis OK
Gross lipemia OK
Gross icterus OK

Specimen Stability Information
Provides a description of the temperatures required to transport a specimen to the performing laboratory, alternate acceptable temperatures are also included

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

Useful For
Suggests clinical disorders or settings where the test may be helpful

Estimating the amount of circulating free thyroxine (free thyroxine index) using the total thyroxine and thyroid binding capacity (T-uptake)

Clinical Information
Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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) has replaced the FTI test in most clinical situations.

Reference Values
Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

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

 

For International System of Units (SI) conversion for Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Interpretation
Provides information to assist in interpretation of the test results

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.

Cautions
Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

This test cannot be used for patients receiving treatment with lipid-lowering agents containing dextrothyroxine (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.

Clinical Reference
Recommendations for in-depth reading of a clinical nature

1. Whitley RJ, Meikle AW, Watts NB. Thyroid function. In: Burtis CA, Ashwood, ER, eds. Tietz Fundamentals of Clinical Chemistry. 4th ed. WB Saunders Company; 1996:645-646

2. Wilson JD, Foster DW, Kronenburg MD, et al. Williams Textbook of Endocrinology. 9th ed. WB Saunders Company; 1998:407-477

3. Freedman DB, Halsall D, Marshall WJ, Ellervik C. Thyroid disorders. In: Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1572-1616

4. Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343-1421

5. Persani L, Cangiano B, Bonomi M. The diagnosis and management of central hypothyroidism in 2018. Endocr Connect. 2019;8(2):R44-R54. doi: 10.1530/EC-18-0515

Method Description
Describes how the test is performed and provides a method-specific reference

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)

PDF Report
Indicates whether the report includes an additional document with charts, images or other enriched information

No

Day(s) Performed
Outlines the days the test is performed. This field reflects the day that the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time before the test is performed. Some tests are listed as continuously performed, which means that assays are performed multiple times during the day.

Monday through Sunday

Report Available
The interval of time (receipt of sample at Mayo Clinic Laboratories to results available) taking into account standard setup days and weekends. The first day is the time that it typically takes for a result to be available. The last day is the time it might take, accounting for any necessary repeated testing.

Same day/1 to 2 days

Specimen Retention Time
Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded

7 days

Performing Laboratory Location
Indicates the location of the laboratory that performs the test

Rochester

Fees
Several factors determine the fee charged to perform a test. Contact your U.S. or International Regional Manager for information about establishing a fee schedule or to learn more about resources to optimize test selection.

  • Authorized users can sign in to Test Prices for detailed fee information.
  • Clients without access to Test Prices can contact Customer Service 24 hours a day, seven days a week.
  • Prospective clients should contact their account representative. For assistance, contact Customer Service.

Test Classification
Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR) product.

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
Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Clinic Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.

CPT codes are provided by the performing laboratory.

84479-Thyroxine binding capacity

84436-Thyroxine total

LOINC® Information
Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the order and results codes of this test. LOINC values are provided by the performing laboratory.

Test Id Test Order Name Order LOINC Value
FRTUP Free Thyroxine Index(FTI), S 32215-6
Result Id Test Result Name Result LOINC Value
Applies only to results expressed in units of measure originally reported by the performing laboratory. These values do not apply to results that are converted to other units of measure.
T4S Thyroxine, Total, S 83119-8
TUPC Thyroxine Binding Capacity, S 74795-6
FRTI Free Thyroxine Index 32215-6

Test Setup Resources

Setup Files
Test setup information contains test file definition details to support order and result interfacing between Mayo Clinic Laboratories and your Laboratory Information System.

Excel | PHP Pdf | CMS Pdf

Sample Reports
Normal and Abnormal sample reports are provided as references for report appearance.

Normal Reports | Abnormal Reports

SI Sample Reports
International System (SI) of Unit reports are provided for a limited number of tests. These reports are intended for international account use and are only available through MayoLINK accounts that have been defined to receive them.

SI Normal Reports | SI Abnormal Reports