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Test ID: FRT3 T3 (Triiodothyronine), Free, Serum

Reporting Name

T3 (Triiodothyronine), Free, S

Useful 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

Clinical 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.


Elevated free T3 (triiodothyronine) values are associated with thyrotoxicosis or excess thyroid hormone replacement.

Report Available

1 to 3 days

Day(s) Performed

Monday through Saturday

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 Unpublished data

Method Name

Immunoenzymatic Assay

Specimen Type


Specimen 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

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


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
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