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

Additional Codes

Ordering Mnemonic
MayoAccess: BCL418

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:  ACMIA / bichromatic rate

Free T3:  LOCI® / Chemiluminescence (flash)


Specimen Requirements

  • Draw blood in a plain, red-top tube(s) or a serum gel tube(s). 
  • Spin down and send 1 mL of serum frozen in plastic vial.


Specimen Type Temperature Time
Serum Refrigerated 2 days
  Frozen (preferred) 1 month


Reject Due To

  • Gross hemolysis
  • 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