T3, Uptake (Triiodothyronine Uptake)

CPT: 84479

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 4 days

Test Details

Additional Information

The thyroid is a small, butterfly-shaped gland at the base of the throat. It makes two hormones, T3 and T4 (thyroxine). Hormones are chemical messengers in the bloodstream that control the actions of certain cells or organs.  

Thyroid hormones work together to control the way the body uses energy. They affect weight, heart, body temperature, muscle strength, and even mood. In children, thyroid hormones affect growth, too.  

The T3 uptake test is used as an indirect measure of thyroid hormone binding capacity. The test is performed by taking a blood sample, to which an excess of radioactive exogenous T3 is added to saturate the transport protein TBG. The performance of a T3 uptake measures the available thyroid hormone binding sites. T3 uptake is not a measurement of serum T3. It should never be used alone; rather, its usual application is used with thyroxine (T4). 

High levels of T3 can indicate hyperthyroidism, caused by Graves’ disease or even by thyroiditis and toxic nodular goiters. In rare cases, high T3 levels can show thyrotoxicosis or thyroid cancer. Hyperthyroidism is often treated with antithyroid medication, radioactive iodine therapy, or thyroidectomy (thyroid surgery).‌ 

Low T3 levels may point to hypothyroidism. Depending on the situation, this can be the result of different medications, recent thyroid surgery, radiation therapy, pregnancy, or iodine deficiency. Another cause of hypothyroidism is Hashimoto’s thyroiditis, which is an autoimmune disorder where the immune system attacks the thyroid gland. 




  1. American Thyroid Association: “Hypothyroidism (Underactive),” “Hyperthyroidism (Overactive),” “Thyroid Function Tests.”‌ 
  2. Indian Journal of Endocrinology and Metabolism: “Does fasting or postprandial state affect thyroid function testing?” 
  3. UCLA Endocrine Center: “What are Normal Thyroid Hormone Levels?” 
  4. University of Rochester Medical Center: “Free and Bound Triiodothyronine (Blood).”  


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.