T3, Free (Free Triiodothyronine)

CPT: 84439



Expected Turnaround Time

24 Hours

Turnaround time is defined as the usual number of days from the date of pickup of a specimen for testing to when the result is released to the ordering provider. In some cases, additional time should be allowed for additional confirmatory or additional reflex tests. Testing schedules may vary.

Specimen Requirements


Serum, Plasma


SST (Serum), Red Top (Separate Serum), Lithium Heparin (Plasma)

Storage Instructions

Room Temp., Refrigerated (2-8 C)

Stability Requirements

Room temp. 72hrs., Refrigerated 7 days

Test Details

Additional Information

T3, also known as triiodothyronine, is a thyroid hormone with multiple functions in the body. T3 regulates the body’s metabolism, activates cells, and enables important organs to function. T3 has three iodine atoms attached to it. Thyroxine (T4), the other primary thyroid hormone, has four iodine atoms. 

 The thyroid produces some of the T3 that circulates in the body. On the other hand, most of it starts as T4 and is changed to T3 in the blood by the removal of one iodine atom. As it circulates in the blood, more than 99% of T3 in the body is bound to proteins. Free T3 refers to the fraction of T3 that is not linked to proteins. The body regulates the interaction between bound and free T3 in an exact balance to ensure optimal physiological functions.  

 The active form of T3 that is not bound to protein is measured in the free T3 test. If a thyroid problem is suspected, blood tests for free T3 and total T3 may be ordered to assess thyroid function. They can also be used to diagnose pituitary gland problems, determine the severity and type of thyroid disease, and monitor the progress of thyroid condition treatment. 

Thyroid function is evaluated by total Free T3 testing. Free and total T3 testing, along with other thyroid function tests, are used to diagnose thyroid disorders such as an overactive or underactive thyroid gland, as well as to assess the severity of thyroid problems, diagnose pituitary gland disorders, and monitor patients with known thyroid conditions. 

The free T3 test is often included in a thyroid panel, a group of tests used to assess thyroid health. If a patient has had an abnormal thyroid-stimulating hormone (TSH) test, T3 testing might be used as a follow-up test. 

Thyroid replacement therapy effectiveness can be determined via free T3 testing. It can also aid in the diagnosis of thyroid hormone-binding protein abnormalities. 




  1. Baskin HJ. Endocrinologic evaluation of impotence. South Med J. 1989; 82(4):446-449. PubMed 2495570  
  1. Brennan MD, Klee GG, Preissner CM, et al. Heterophilic serum antibodies: A cause for falsely elevated serum thyrotropin levels. Mayo Clin Proc. 1987; 62(10):894-898. PubMed 3657306  
  1. Clark PM, Clark JD, Holder R, et al. Pulsatile secretion of TSH in healthy subjects. Ann Clin Biochem. 1987; 24(Pt 5):470-476. PubMed 3310836  


Statement on Medical Necessity
All ordered tests should be medically necessary for the diagnosis or detection of disease, illness, impairment, symptom, syndrome, or disorder and the results should be used in the medical management and treatment decisions for the patient. Solaris requires ICD-10 codes with each order for lab testing and both the tests ordered and the diagnosis should be documented in the provider’s medical record for the patient. The United States Department of Health and Human Services, Office of Inspector General, takes the position that a provider who orders medically unnecessary tests may be subject to civil penalties.

Panels and Profiles
Solaris offers Providers the convenience of ordering test combinations in a group at times with the flexibility to choose appropriate test(s) for individual patients. Providers should only order those tests that he or she believes are medically necessary for each patient, and a lesser inclusive profile or individual tests should be ordered if not all tests in the test combination/profile are medically necessary. All tests offered in a test combination/profile may be ordered separately as individual tests. Solaris encourages clients to contact their Solaris representative if the testing configurations shown do not meet individual needs for any reason, or if some other combination of procedures is needed.

CPT Codes
CPT Codes listed are in accordance with Current Procedural Terminology, a publication of the American Medical Association. CPT codes are the responsibility of the billing party and are listed here for informational purposes. Correct coding may vary from one carrier to another. Solaris may bill specific carriers using codes other than what is shown.


For questions or inquiries related to testing please reach out to
customerservice@solarisdx.com or contact us by phone at (844) 550-0308.