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Free T3 Blood Test

Last updated Oct. 11, 2015


What are the other Names for this Test? (Equivalent Terms)

  • Free Triiodothyronine Blood Test
  • FT3 Blood Test 

What is Free T3 Blood Test? (Background Information)

  • Triiodothyronine (T3) is a hormone produced by the thyroid gland. It stimulates nearly every cell in the body to increase oxygen and energy consumption. T3 increases body metabolism, causing increased growth and development
  • The thyroid gland is situated in the front of the throat, just below the cartilage of the Adam’s apple. Apart from T3, the thyroid gland also produces two other hormones namely, T4 or thyroxine (also called tetraiodothyronine), and calcitonin
  • The pituitary gland stimulates the thyroid gland, through production of thyroid-stimulating hormone (TSH). TSH production is in turn stimulated by thyrotropin-releasing hormone (TRH), from the hypothalamus of the brain
  • The system operates via a negative feedback loop; TSH stimulates thyroid gland hormone production, whose increase then inhibits TRH production by the brain. This results in decreased stimulation of TSH production, leading to decreased thyroid gland hormones
  • Calcitonin is made by C cells. It plays a role in decreasing calcium levels in blood, by stimulating calcium excretion by the kidneys, and through calcium uptake by the bones
  • The difference between T3 and T4 is that T3is 4-5 times more effective than T4 at increasing cellular metabolism. However, nearly 90% of T3 that reaches the cells, is converted from T4
  • Thus, in other words, the thyroid gland produces larger quantities of T4 relative to T3 - roughly in a 10:1 ratio. But, T3 is primarily responsible for causing cell growth and development
  • The reason for this conversion step is to add an opportunity for the body to regulate and fine-tune its metabolic rate. If the rate of T4 to T3 conversion is decreased, cellular metabolism can be lowered, without having to synthesize more hormones
  • T3 and T4 are carried by binding proteins. Only, less than 1% of T3 and T4 circulate freely in blood. These binding proteins include:
    • Thyroid-binding globulins (TBGs), which carry 70% of T4 and 75% of T3
    • Transthyretin, also known as thyroid-binding prealbumin (TBPA), and
    • Albumin, a plasma protein
  • Thyroid hormones bound to proteins, serve as the body’s reserves. At any time, more than 1 week’s supply of T3 and T4 are bound by proteins
  • As cells take up free hormones, bound hormones are released and become free. This is because of the equilibrium that exists between free and bound hormones, which adds yet another opportunity for regulation
  • The result of this equilibrium between free and bound thyroid hormones is that levels of free T3 and free T4 directly relate to levels of bound and total T3 and T4
  • As their names suggest, T3 and T4 are made using iodine. Without iodine, the production of these hormones stop, or is reduced. This can lead to the formation of a goiter (swelling) in the neck as levels of fluid and hormone precursors accumulate
  • The Free T3 Blood Test is a test to assess the levels of unbound T3 in blood. This test is unaffected by the levels of binding proteins. It is used to diagnose T3 toxicity, and to aid other thyroid tests 

What are the Clinical Indications for performing the Free T3 Blood Test?

Following are the clinical indications for performing a Free T3 Blood Test:

  • As a follow-up to TSH and free T4 tests
  • Monitoring thyroid hormone therapy
  • Early or late onset of puberty
  • Distinguishing between primary and secondary hypogonadism
  • Masculinization in females, such as excessive hair growth
  • Infertility
  • Decreased sex drive
  • Erectile dysfunction
  • Diminished menstrual cycle
  • Abnormal uterine bleeding 

How is the Specimen Collected for Free T3 Blood Test?

Following is the specimen collection process for Free T3 Blood Test:

Sample required: Blood

Process: Insertion of a needle into an arm vein.

Preparation required: No special preparation is needed prior to the test. 

What is the Significance of the Free T3 Blood Test Result?

The significance of Free T3 Blood Test is explained:

  • Increased free T3 may indicate:
    • Hyperthyroidism
    • T3 toxicosis
  • Decreased T3 may indicate:
    • Hypothyroidism
    • Non-thyroidal chronic diseases
    • Malnutrition 

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. 

Additional and Relevant Useful Information:

  • Certain factors interfere with the results of the Free T3 Blood Test. These include altitude, pregnancy, age, strenuous exercise, and nutritional status 

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis. 

What are some Useful Resources for Additional Information?

DoveMed is currently working to bring you additional resources.

Please sign up by creating a DoveMed account to receive periodic notification on information updates. 

References and Information Sources used for the Article:

Martini, F., Nath, J. L., & Bartholomew, E. F. (2012). Fundamentals of anatomy & physiology (9th ed.). San Francisco: Benjamin Cummings.

Schnell, Z. B., Van, L. A., & Kranpitz, T. R. (2003). Davis's Comprehensive handbook of laboratory and diagnostic tests: With nursing implications. Philadelphia: F.A. Davis.

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Oct. 18, 2014
Last updated: Oct. 11, 2015