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Test Code T3FR T3 (Triiodothyronine), Free, Serum


Specimen Required


Supplies: Sarstedt Aliquot Tube, 5 mL (T914)

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions: Centrifuge and aliquot serum into a plastic vial.


Secondary ID

621321

Useful For

A second- or third-level test of thyroid function

 

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

Method Name

Electrochemiluminescence Immunoassay (ECLIA)

Reporting Name

T3 (Triiodothyronine), Free, S

Specimen Type

Serum

Specimen Minimum Volume

0.75 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days
  Frozen  30 days

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

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).

Reference Values

Pediatric

0-1 month: 2.7-8.5 pg/mL

1-<12 months: 3.4-5.6 pg/mL

1-<14 years: 3.0-5.1 pg/mL

14-<19 years: 3.3-5.3 pg/mL

 

Adult (≥19 years): 2.0-4.4 pg/mL

 

For International System of Units (SI) conversion for Reference Values, see www.mayocliniclabs.com/order-tests/si-unit-conversion.html.

Interpretation

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

Method Description

The Roche Elecsys FT3 III (free T3 [triiodothyronine]) is a competitive assay using electrochemiluminescence detection. The patient sample is incubated with a sheep polyclonal anti-T3 antibody labeled with ruthenium. Streptavidin-coated microparticles and biotinylated T3 are added for a second incubation during which the still free binding sites of the labeled antibody become occupied. The resulting immunocomplex becomes bound to the solid phase by interaction of biotin and streptavidin. The reaction mixture is aspirated into the measuring cell where the microparticles are magnetically captured onto the surface of the electrode. Unbound substances are removed and application of a voltage to the electrode induces the electrochemiluminescent emission. This signal is measured against a calibration curve to determine patient results.(Package insert: Elecsys FT3 III. Roche Diagnostics; 10/2022)

Day(s) Performed

Monday through Saturday

Performing Laboratory

Mayo Clinic Laboratories in Rochester

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
T3FR T3 (Triiodothyronine), Free, S 3051-0

 

Result ID Test Result Name Result LOINC Value
T3FR T3 (Triiodothyronine), Free, S 3051-0