T4, Free (Free Thyroxine)

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

T4 free Thyroxine is one of the two hormones produced by the thyroid gland. Mostly thyroid produces hormones in the form free T4. Since this hormone is relatively inactive, it is converted into the much more active T3 in the liver and other tissues. Together with TSH, free T4 gives clues on how the thyroid is functioning.  

Thyroxine (T4) is an important hormone secreted by the thyroid into the blood. T4 is generally of two forms: free T4 and bound T4. Free T4 (fT4) circulates freely through the tissues whereas bound T4 is found attached to the proteins, thereby, preventing it from entering the tissues. Over 99% of T4 are said to be in bound form. Since the free form is a biologically active form that is made more available for the tissues to perform its stipulated functions, accuracy can be achieved with free T4 than by testing the bound T4. 

T4 is secreted in its inactive form at first which gets converted to an active form called triiodothyronine (T3) to facilitate its functions which include maintaining the metabolic rate, muscle control, cardiac and digestive functions, brain development and bone maintenance. This is made possible with the help of organs such as the liver and kidney. 


  1. American Thyroid Association [Internet]. Falls Church (VA): American Thyroid Association; c2022. Thyroid Function Tests [cited 2022 Apr 5]; [about 7 screens]. Available from: https://www.thyroid.org/thyroid-function-tests 
  2. Hinkle J, Cheever K. Brunner & Suddarth’s Handbook of Laboratory and Diagnostic Tests. 2nd Ed, Kindle. Philadelphia: Wolters Kluwer Health, Lippincott Williams & Wilkins; c2014. Thyroxine, Serum 485 p. 
  3. Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co. Inc.; c2022. Overview of the Thyroid Gland [modified 2020 Oct; cited 2022 Apr 5]; [about 4 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/thyroid-gland-disorders/overview-of-the-thyroid-gland 

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.