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Test ID FRT3 T3 (Triiodothyronine), Free, Serum

Reporting Name

T3 (Triiodothyronine), Free, S

Useful 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

Serum


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.

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 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 https://www.mayocliniclabs.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, approved or is exempt 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 83127-1

 

Result ID Test Result Name Result LOINC Value
FRT3C T3 (Triiodothyronine), Free, S 83127-1

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 Unpublished data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 day

Clinical 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