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HelpTest ID FRT3 T3 (Triiodothyronine), Free, Serum
Reporting Name
T3 (Triiodothyronine), Free, SUseful For
Free triiodothyronine (T3) is a second- or third-level test of thyroid function; it provides further confirmation of hyperthyroidism, supplementing the tetraiodothyronine (T4), sensitive thyrotropin (sTSH), and total T3 assays
Evaluating clinically euthyroid patients who have an altered distribution of binding proteins
Monitoring thyroid hormone replacement therapy
Specimen Type
SerumSpecimen Required
Patient Preparation: For 12 hours before this test do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.
Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Specimen Volume: 0.6 mL
Specimen Minimum Volume
0.5 mL
Specimen Stability Information
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum | Refrigerated (preferred) | 7 days |
| Frozen | 90 days |
Reference Values
≥1 year: 2.8-4.4 pg/mL
For SI unit Reference Values, see https://www.mayomedicallaboratories.com/order-tests/si-unit-conversion.html.
Day(s) and Time(s) Performed
Monday through Friday; 5 a.m.-12 a.m.
Saturday; 6 a.m.-6 p.m.
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
84481
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| FRT3 | T3 (Triiodothyronine), Free, S | 3051-0 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| FRT3C | T3 (Triiodothyronine), Free, S | 3051-0 |
Interpretation
Elevated free triiodothyronine (FT3) values are associated with thyrotoxicosis or excess thyroid hormone replacement.
Clinical Reference
1. Demers LM, Spencer Cl: The thyroid: pathophysiology and thyroid function testing. In Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Fourth edition. Edited by CA Burtis, ER Ashwood, DE Bruns. St. Louis, Elsevier Saunders Company. 2006, pp 2053-2087
2. FT3 Validation 2005 and AIA Retrospective Validation V-139, 2009. Unpublished data
Analytic Time
Same day/1 dayClinical Information
Normally triiodothyronine (T3) circulates tightly bound to thyroxine-binding globulin and albumin. Only 0.3% of the total T3 is unbound (free); the free fraction is the active form.
In hyperthyroidism, both thyroxine (tetraiodothyronine; thyroxine: T4) and T3 levels (total and free) are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated. Generally, free T3 (FT3) measurement is not necessary since total T3 will suffice. However, FT3 levels may be required to evaluate clinically euthyroid patients who have an altered distribution of binding proteins (eg, pregnancy, dysalbuminemia).
Some investigators recommend the FT3 assay for monitoring thyroid replacement therapy, although its clinical role is not precisely defined.
Method Name
Immunoenzymatic Assay