Test ID: FRT3P
T3 (Triiodothyronine), Free and Total, 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
Triiodothyronine (T3) is a second-order test in follow-up to low thyroid stimulating hormone values in the evaluation of patients suspected of having hyperthyroidism caused by excess T3 (T3 toxicosis).
Free T3 (FT3) levels can be helpful in evaluating patients with altered levels of binding proteins, such as pregnant patients, patients receiving estrogens and anabolic steroids, and patients with dysalbuminemia. Some investigators recommend FT3 for monitoring thyroid replacement therapy, although its clinical role is not precisely defined. FT3 also provides a further confirmatory test for hyperthyroidism to supplement the thyroxine (T4), sensitive thyrotropin, and total T3 assays.
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 |
|---|---|---|---|
| FRT3 | T3 (Triiodothyronine), Free, S | Yes | Yes |
| T3 | T3 (Triiodothyronine), Total, S | Yes | Yes |
Method Name A short description of the method used to perform the test
Immunoenzymatic Assay
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
FT3 and TT3
T3, Free & Total, Serum
Total & Free T3, Serum
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: 1 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 reject |
| Lipemia | Mild OK; Gross OK |
| 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 | 90 days |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Thyroid hormones regulate a number of developmental, metabolic, and neural activities throughout the body. The 2 main hormones secreted by the thyroid gland, under the influence of thyroid stimulating hormone, are thyroxine (T4), which contains 4 atoms of iodine, and triiodothyronine (T3). T3 is also produced by conversion (deiodination) of T4 in peripheral tissues; 20% of T3 occurs from direct synthesis and 80% occurs from conversion of T4 to T3. Circulating levels of T3 are much lower than T4 levels, but T3 is more metabolically active than T4, although its effect is briefer.
In circulation, 99.7% of T3 is bound to proteins, primarily thyroxine-binding globulin. Free T3 is the metabolically active form of T3 and accounts for 0.3% of the total T3.
In hyperthyroidism, both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated.
In hypothyroidism, T4 and T3 levels are decreased. T3 levels are also often low in sick or hospitalized euthyroid patients.
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.
T3, FREE
> or =1 year: 2.0-3.5 pg/mL
T3, TOTAL
> or =1 year: 80-190 ng/dL
Interpretation Provides information to assist in interpretation of the test results
Triiodothyronine (T3) values >180 ng/dL in adults or >200 ng/dL in children are consistent with hyperthyroidism or increased thyroid hormone binding proteins.
T3 often is low in sick or hospitalized patients, so it is not a good indicator of hypothyroidism.
Abnormal levels of binding proteins to T3 (primarily albumin and thyroxine-binding globulin) may cause either high or low T3 concentrations in euthyroid patients.
Elevations in free T3 are associated with thyrotoxicosis or excess thyroid replacement therapy.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Triiodothyronine (T3) is not a reliable marker for hypothyroidism and is not useful for general screening of the population without a clinical suspicion of hyperthyroidism.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Hay ID, Klee GG: Linking medical needs and performance goals: clinical and laboratory perspectives on thyroid disease. Clin Chem 1993;39:1519-1524
2. Klee GG: Clinical usage recommendations and analytic performance goals for total and free triiodothyronine measurements. Clin Chem 1996;42:155-159
Method Description Describes how the test is performed and provides a method-specific reference
The instrument used is a Beckman Coulter DXI 800. The access total and free triiodothyronine (T3) assays are competitive-binding immunoenzymatic assays. For the total T3 assay, sample is added to a reaction vessel with a stripping agent to dissociate T3 from the binding proteins. T3 in the sample competes with the T3 analogue coupled to biotin for anti-T3 alkaline Phosphatase conjugate. Of the resulting antigen:antibody complexes, the T3 analogue:antibody complexes are bound to the streptavidin-coated solid phase. Separation in a magnetic field and washing removes the sample T3:antibody complexes and other materials not bound to the solid phase.
For the Free T3 (FT3) assay, sample is added to a reaction vessel with mouse monoclonal antitriiodothyronine (anti-T3) and paramagnetic particles coated with goat antimouse capture antibody. During the first incubation, free T3 in the sample reacts with the anti-T3 antibody and is subsequently bound to the capture antibody. After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field, while unbound materials are washed away. Next, T3-alkalinephosphatase conjugate and buffer are added to the reaction vessel to react with the remaining anti-T3 antibody binding sites. After incubation in a reaction vessel, materials bound to the solid phase are held in a magnetic field, while unbound materials are washed away. For both the total T3 and the FT3 assays, the chemiluminescent substrate Lumi-Phos* 530 is added to the vessel and light generated by the reaction is measured with a luminometer. The light production is inversely proportional to the concentration of total and FT3 in the sample. The amount of analyte in the sample is determined from a stored, multipoint calibration curve. (Package Insert: Beckman Coulter Ireland Inc. Ireland, 2005)
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; 3rd shift
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.
84480-T3, total
84481-T3, free
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 |
|---|---|---|
| T3 | T3 (Triiodothyronine), Total, S | 3053-6 |
| FRT3C | T3 (Triiodothyronine), Free, S | 3051-0 |
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