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Test Code Billings Clinic: 7684 T3 (Triiodothyronine), Free and Total, Plasma or Serum

Important Note

Although the Free T3 assay is FDA Approved for other patient populations the assay performance characteristics have not been established for neonates.

Performing Laboratory

Billings Clinic Laboratory: Chemistry

Useful For

Free T3

  • 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

Total T3

  • Second-order testing for hyperthyroidism in patients with low thyroid-stimulating hormone values and normal thyroxine levels
  • Diagnosis of triiodothyronine toxicosis


Total T3:  Direct Chemiluminescence

Free T3:   Direct Chemiluminescence


Specimen Requirements

Submit only 1 of the following specimens:

Serum or plasma should be physically separated from cells as soon as possible with a maximum limit of two hours from the time of collection.



  • Draw blood in a green-top (lithium heparin) tube(s).
  • Spin down and send 1.0 mL of heparinized plasma frozen.



  • Draw blood in a plain, red-top tube(s) or a serum gel tube(s).
  • Spin down and send 1.0 mL of serum frozen


Specimen Type Temperature Time
Plasma, Serum Refrigerated 2 days
  Frozen (preferred) 1 month
  Ambient 8 hours


Reject Due To

  • Gross hemolysis
  • Specimens with apparent contamination
  • Specimens stored at room temperature greater than 8 hours
  • Samples frozen and thawed more than once

Reference Values

See individual test listings

Day(s) Test Set Up


Test Classification and CPT Coding

84480 - total

84481 - free

LOINC Code Information

Reporting Name LOINC Code
Free T3 3051-0
Total T3 3053-6