Update on Free T3 Testing

There are a few circumstances that may affect Free T3 (triiodothyronine) test results.  The following two test descriptions from LabCorp, a major nationwide diagnostic laboratory, indicate conditions which may interfere with testing and when the Dialysis and LC/MS-MS may be preferred. 

  • Biotin may interfere with Free T3 and other thyroid tests. 
  • Thyroid antibodies may interfere with free hormone immunoassays.
  • Pregnancy, oral contraceptives, hormone therapy, severe illness and other conditions which might affect binding proteins — dialysis and MS/MS preferred.

If Free T3 immunoassay test results, along with TSH and Free Tdon’t appear to fit your clinical picture, talk with your provider about using the dialysis and MS/MS method instead.  This test requires a frozen sample and has a longer turn around time but is less subject to interference.  There is no reason to think Free T3 tests are not reliable for thyroid cancer patients as long as potentially interfering conditions are taken into account. 

LabCorp:Triiodothyronine (T3) Free
Test 010389  CPT: 84481  
Free T3, T3, Free
Expected Turnaround Time
1 – 2 days
Room temperature
Serum or Plasma
Evaluate thyroid function and assess abnormal binding protein disorders
Circulating antibodies to thyroid hormones have also been reported to interfere with free hormone immunoassays.
Electrochemiluminescence immunoassay (ECLIA)
Special Instructions
This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

LabCorp:Triiodothyronine (T3), Free, Dialysis and LC/MS-MS (Endocrine Sciences
Test 503600  CPT: 84481  
Free T3, Free Triiodothyronine (T3), FT3
Expected Turnaround Time
6-10 days
Serum (preferred) or plasma, frozen
Evaluate thyroid function and assess abnormal binding protein disorders
Equilibrium dialysis and high-pressure liquid chromatography/tandem mass spectrometry (HPLC/MS-MS)
Additional Information
Direct measurement of FT3 by equilibrium dialysis, high-pressure liquid chromatography, and tandem mass spectrometry (ED-HPLC/MS-MS) provides enhanced accuracy over commonly used analog (automated) methods and is not affected by clinical conditions that impact protein binding capacity.1-4 Examples of clinical conditions in which the FT3 by dialysis and MS/MS method may be preferred include congenital absence of thyroxine-binding protein (TBG);1 pregnancy;1-5 administration of oral contraceptives or hormone therapy;6 administration of atypical antipsychotic medications;7 malignancy or other critical illnesses.8