Test ID: FRTIX
Free Thyroxine Index (FTI), Serum
Secondary ID 
A test code used for billing and in test definitions created prior to November 2011
NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.
Useful For Suggests clinical disorders or settings where the test may be helpful
T-Uptake (TU) and total thyroxine (T4) are used to estimate the amount of circulating free T4. The estimate, or the free thyroxine index (FTI), is a normalized measurement that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins. Hyperthyroidism causes increased FTI and hypothyroidism causes decreased values. TU results are changed by drugs or physical conditions that alter the patient's thyroxine-binding globulin levels, or drugs that compete with endogenous T4 and triiodothyronine (T3) for protein-binding sites. When serum contains high levels of T3 and T4 as in hyperthyroidism, fewer unoccupied binding sites are available. Conversely, in hypothyroidism, more binding sites are available.
Method Name A short description of the method used to perform the test
Chemiluminometric Immunoassay
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Aliases Lists additional common names for a test, as an aid in searching
Thyroxine Index
Specimen Type Describes the specimen type needed for testing
Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.
Container/Tube:
Preferred: Red top
Acceptable: Serum gel
Specimen Volume: 0.5 mL
Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.
Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected
| Hemolysis | Mild OK; Gross OK |
| Lipemia | NA |
| Icterus | NA |
| Other | NA |
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum | Refrigerated (preferred) | 7 days |
| Frozen | 30 days |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
The T-Uptake is an assessment of unsaturated thyroid binding proteins in serum. The free thyroxine index is an estimate of the amount of circulating free thyroxine.
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.
T-UPTAKE
Males: 27-37%
Females: 20-37%
FREE THYROXINE INDEX
Males
<1.5 suggest hypothyroidism
1.5-3.0 suggest euthyroidism
>3.0 suggest hyperthyroidism
Females
<1.3 suggest hypothyroidism
1.3-3.0 suggest euthyroidism
>3.0 suggest hyperthyroidism
THYROXINE, TOTAL
Males
0-11 months: not established
1-9 years: 6.0-12.5 mcg/dL
10-23 years: 5.0-11.0 mcg/dL
> or =24 years: 5.0-12.5 mcg/dL
Females
0-11 months: not established
1-9 years: 6.0-12.5 mcg/dL
10-17 years: 5.0-11.0 mcg/dL
> or =18 years: 5.0-12.5 mcg/dL
Interpretation Provides information to assist in interpretation of the test results
The T-Uptake (TU) values used are interpreted in conjunction with total thyroxine (T4) measurements. The free thyroxine index is calculated as:
-FTI=(T4 concentration) x (% TU)/100
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Direct measurement of free thyroxine (T4) has replaced the T-Uptake (TU) test in most clinical situations.
TU values are a function of thyroid hormone binding capacity of serum. Some conditions may result in abnormal TU values even when no thyroid malfunction is present. The conditions listed below may alter TU values.
Conditions that decrease TU values:
-Pregnancy (especially in the last trimester)
-Elevated estrogen levels
-Acute hepatitis
-Drugs which prevent ovulation
Conditions that elevate T-Uptake values:
-Protein malnutrition
-Chronic liver disease
-Nephrotic syndrome
-Phenytoin therapy
-Heparin therapy
-Uremia
-Large doses of salicylates
-Antibiotics
Thyroid preparations that maintain normal triiodothyronine (T3) and T4 concentrations can elevate T-Uptake values when the dose hormone is excessive and decrease TU values when the dose is inadequate. Replacement therapy with T3 results in decreased TU values.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Whitley RJ, Meikle AW, Watts NB: Thyroid Function. In Tietz Fundamentals of Clinical Chemistry. Fourth Edition. Edited by CA Burtis, ER Ashwood. Philadelphia, EB Saunders Company 645-646, 1996
2. Klee GG, Hinz VS: The Ciba Corning ACS:180 Plus. In Immunoassay Automation: An Updated Guide to Systems. Edited by DW Chan, Associated Press, New York 63-102, 1996
Method Description Describes how the test is performed and provides a method-specific reference
This is a chemiluminometric immunoassay. T-Uptake (TU) is measured using an automated sandwich two-site chemiluminometric immunoassay on the Advia: Centaur instrument. The specimen is incubated with acridinium ester (AE)-labeled triiodothyronine (T3)-BGG (bovine gamma globulin) and unlabeled triiodothyronine (T3). The unlabeled T3 fills available thyroid-binding sites in the specimen; and the AE-labeled T3-BGG does not bind to the binding proteins in the specimen. The AE-labeled T3-BGG and the remaining unlabeled T3 then compete for binding sites on a monoclonal mouse anti-T3 antibody in the solid phase. A greater amount of unlabeled T3 bound to the binding proteins in the specimen results in more T3-BGG-AE binding to the monoclonal antibody, an indication of a greater amount of unsaturated binding proteins. A direct relationship exists between the concentration of unsaturated binding proteins in a specimen and the relative light units (RLU’s) detected by the system. (Package insert: Bayer Advia: Centaur TUP. Bayer Diagnostics, East Walpole, MA 02032)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Monday through Friday, Sunday; 6 a.m.-3 p.m.
Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.
Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result
Specimen Retention Time Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded
Performing Laboratory Location The location of the laboratory that performs the test
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's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.
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 Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.
84436-Thyroxine, total
84479-T3 Uptake
LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.
| Result ID | Reporting Name | LOINC Code |
|---|---|---|
| T4S | Thyroxine, Total, S | 3026-2 |
| TUP | T3 (Triiodothyronine), Uptake, S | 3050-2 |
| FRTI | Free Thyroxine Index | 32215-6 |
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