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Also known as: Total T3; Free T3; FT3
Formal name: Triiodothyronine

At a Glance

Why Get Tested?

To help diagnose hyperthyroidism

When to Get Tested?

When a person has an abnormal TSH or T4 result or has symptoms of hyperthyroidism

Sample Required?

A blood sample taken from a vein in the arm

Test Preparation Needed?

None needed; however, certain medications can interfere with the T3 test, so tell your doctor about any drugs that you are taking.

The Test Sample

What is being tested?

This test measures the amount of triiodothyronine, or T3, in the blood. T3 is one of two major hormones produced by the thyroid gland; the other is called thyroxine, or T4. The thyroid gland is a small butterfly-shaped organ that lies flat across the windpipe. The hormones it produces control the rate at which the body uses energy. Their production is regulated by a feedback system. When blood levels of thyroid hormones decline, the hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary gland to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and/or release more thyroid hormones. Most of the thyroid hormone produced is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.

If the thyroid gland produces excessive amounts of T4 and T3, then the person affected may have symptoms associated with hyperthyroidism, such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes. In some cases, the person's eyes cannot move normally and they may appear to be staring. In other cases, the eyes may appear to bulge.

If the thyroid gland produces insufficient amounts of thyroid hormones, then the person may have symptoms associated with hypothyroidism and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. Blood levels of hormones may be increased or decreased because of insufficient or excessive production by the thyroid gland, due to thyroid dysfunction, or due to insufficient or excessive TSH production related to pituitary dysfunction.

About 99.7% of the T3 found in the blood is attached to a protein (primarily thyroxine-binding globulin but also several other proteins) and the rest is free (unattached). Separate blood tests can be performed to measure either the total (both bound and unattached) or free (unattached) T3 hormone in the blood.

How is the sample collected for testing?

A blood sample is obtained from a needle placed in a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. Certain medications can interfere with the T3 test, so tell the doctor about any drugs that you are taking.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Eckman, A. (Updated 2010 April 20). T3 Test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed February 2011.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 976-977.

Aytug, S. and Shapiro, L. (Updated 2010 April 5). Euthyroid Sick Syndrome. eMedicine [On-line information]. Available online at through Accessed February 2011.

(© 2008). Thyroid Function Tests. American Thyroid Association [On-line information]. Available online at through Accessed February 2011.

Kansagra, S. et. al. (2010 June 10). The Challenges and Complexities of Thyroid Hormone Replacement. Medscape Today from Laboratory Medicine. 2010;41(6):229-348 [On-line information]. Available online at through Accessed February 2011.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Brown, T. (2004 January 27, Updated). T3. MedlinePlus Medical Encyclopedia [On-line information]. Available online at

(© 2005). Triiodothyronine. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at through

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp. 1076-1081.

ARUP's Laboratory Test Directory: T3 Uptake. Available online at through

Amarillo Medical Specialists. How to interpret your blood test results. Available online at through

MedlinePlus Medical Encyclopedia. T3RU test (Updated 10/24/07). Available online at

Shomon, Mary. Thyroid Blood Tests. Guide. Updated March 27, 2007. Available online at through Accessed June 22, 2010.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. St. Louis: Elsevier Saunders; 2006, Pp. 2053, 2063-2064.

Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds (2005). Harrison's Principles of Internal Medicine, 16th Edition, McGraw Hill, Pg 2119.

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