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HelpTest ID FRT3 T3 (Triiodothyronine), Free, Serum
Reporting Name
T3 (Triiodothyronine), Free, SUseful For
Free T3 (triiodothyronine) is a second- or third-level test of thyroid function; it provides further confirmation of hyperthyroidism, supplementing the T4 (tetraiodothyronine), sensitive thyrotropin, 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 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.
Collection Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Submission Container/Tube: Plastic vial
Specimen Volume: 0.6 mL
Collection Instructions: Centrifuge and aliquot serum into plastic vial.
Specimen Minimum Volume
0.5 mL
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 7 days | |
| Frozen | 90 days |
Reference Values
≥1 year: 2.8-4.4 pg/mL
For SI unit Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html
Day(s) Performed
Monday through Saturday
Test Classification
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
84481
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| FRT3 | T3 (Triiodothyronine), Free, S | 83127-1 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| FRT3C | T3 (Triiodothyronine), Free, S | 83127-1 |
Interpretation
Elevated free T3 (triiodothyronine) values are associated with thyrotoxicosis or excess thyroid hormone replacement.
Clinical Reference
1. Welsh KJ, Soldin SJ: DIAGNOSIS OF ENDOCRINE DISEASE: How reliable are free thyroid and total T3 hormone assays?. Eur J Endocrinol. 2016;175(6):R255-R263. doi: 10.1530/EJE-16-0193
2. FT3 Validation 2005 and AIA Retrospective Validation V-139, 2009. Unpublished data
Report Available
1 to 3 daysClinical Information
Normally T3 (triiodothyronine) 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 T4 (thyroxine, tetraiodothyronine) 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