Sign in →

Test Code FRT3 or LAB137 Free T3, Serum or Plasma

Important Note

This test is recommended over Total T3 because Free T3 represents the physiologically active hormone rather than the quantity of carrier proteins.

Specimen Requirements

Container Type:

Optimal: Gold or Mint*

*For STAT requests

Acceptable: Red or Green
Optimal Collection Volume: 

4.5mL; full tube
Minimum Volume:

1 mL whole blood
Collection Instructions:

Gently invert the sample 5-6 times after collection.
Specimen Transport:

Refrigerate
Processing Instructions: 

1. If serum, allow the specimen to clot for 30 minutes.

2. Centrifuge specimen within 2 hours of collection.

3. After centrifugation, specimen tubes without a gel barrier should have the serum or plasma aliquoted to a false bottom container.

4. Keep serum or plasma refrigerated until testing can be performed.

Specimen Stability

Specimen Type Temperature Time
Serum* Refrigerated 7 days
Room Temperature 5 days
Frozen 30 days

*Note: Heparinized plasma in gel separator tubes is less stable than serum and is only suitable for add-on testing up to 2 days following sample collection.

Specimen Rejection Criteria

Hemolysis Mild OK; Moderate OK; Gross Reject
Lipemia Mild OK; Moderate OK; Gross Reject
Icterus Mild OK; Moderate OK; Gross Reject
Other Quantity not sufficient

Useful For

Determination of Free T3 concentration is important for the diagnostic differentiation of euthyroid, hyperthyroid, and hypothyroid states.

Methodology

Roche Cobas - Electrochemiluminescence assay

Reference Ranges

2.00-4.40 pg/mL

 

Note: The reference range above went into effect on 1/5/2022. For details about this change, please see the December 2021 LabWire.

Interpretive

In hyperthyroidism, serum T3 concentrations generally correlate with the serum T4 concentrations, but a small subset of patients has increased T3 only (T3 toxicosis), and therefore T3 should be evaluated in the assessment of patients with suspected hyperthyroidism who have normal Free T4. T3 toxicosis should be suspected if the TSH is decreased, and the FT4 is normal.

Day(s) Performed

24/7

Expected TAT

Same day

Performing Laboratory

Bronson Laboratory, Chemistry - Kalamazoo and Battle Creek

Sample Retention Time

7 days

CPT Code

84481

LOINC Code

3051-0