At a Glance
Why Get Tested?
To help evaluate thyroid gland function and to help diagnose thyroid disorders
When to Get Tested?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None needed; however, certain medications can interfere with the tests included in the panel, so tell your health practitioner about any drugs that you are taking.
The Test Sample
What is being tested?
A thyroid panel is a group of tests that may be ordered together to help evaluate thyroid gland function and to help diagnose thyroid disorders. The tests included in a thyroid panel measure the amount of thyroid hormones in the blood. These hormones are chemical substances that travel through the blood and control or regulate the body's metabolism—how it functions and uses energy.
The thyroid panel usually includes:
- TSH (thyroid-stimulating hormone) – to test for hypothyroidism, hyperthyroidism and to monitor treatment for a thyroid disorder
- Free T4 (thyroxine) – to test for hypothyroidism and hyperthyroidism; may also be used to monitor treatment
- Free T3 or total T3 (triiodothyronine) – to test for hyperthyroidism; may also be used to monitor treatment
Sometimes a T3 resin uptake (T3RU) test is included to calculate, along with the T4 value, the free thyroxine index (FTI), another method for evaluating thyroid function that corrects for changes in certain proteins that can affect total T4 levels.
TSH is produced by the pituitary gland and is part of the body's feedback system to maintain stable amounts of the thyroid hormones T4 and T3 in the blood. When thyroid hormone levels decrease, the pituitary is stimulated to release TSH. TSH in turn stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, thyroid production turns on and off to maintain constant blood thyroid hormone levels.
T3 and T4 are the two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. Together they help control the rate at which the body uses energy. Almost all of the T3 and T4 circulating in the blood is bound to protein. The small portions that are not bound or "free" are the biologically active forms of the hormones. Tests can measure the amount of free T3 or free T4 or the total T3 or total T4 (bound plus unbound) in the blood.
The total T4 and total T3 tests have been used for many years, but they can be affected by the amount of protein available in the blood to bind to the hormone. The free T4 and free T3 tests are not affected by protein levels and are thought by many to be more accurate reflections of thyroid hormone function. In most cases, the free T4 test has replaced that of the total T4 test. However, some professional guidelines recommend the total T3 test, so either total T3 or free T3 test may be used to assess thyroid function.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into 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 tests included in the panel, however, so tell the health practitioner about any drugs being taken.
Ask a Laboratory Scientist
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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
(Updated 2014 May 14). Thyroid Tests. National Endocrine and Metabolic Diseases Information Service [On-line information]. Available online at http://endocrine.niddk.nih.gov/pubs/thyroidtests/index.aspx through http://endocrine.niddk.nih.gov. Accessed January 2015.
(2012 June 4). Thyroid Function Tests. American Thyroid Association [On-line information]. Available online at http://www.thyroid.org/blood-test-for-thyroid/ through http://www.thyroid.org. Accessed January 2015.
(© 1995–2015). Thyroid Function Cascade, Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/83633 through http://www.mayomedicallaboratories.com. Accessed January 2015.
Meikle, A. W. and Straseski, J. (Updated 2014 July). Thyroid Disease. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/ThyroidDz.html?client_ID=LTD through http://www.arupconsult.com. Accessed January 2015.
Clarke, W., Editor (© 2011). Contemporary Practice in Clinical Chemistry 2nd Edition: AACC Press, Washington, DC. Pp 431-449.
Sources Used in Previous Reviews
American Thyroid Association. Thyroid Function Tests, patient information. PDF available for download at http://www.thyroid.org/patients/brochures/FunctionTests_brochure.pdf through http://www.thyroid.org/. Accessed May 2008.
Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006.
(April 27, 2007) MedlinePlus, Medical Encyclopedia. Thyroid Function Tests. Available online at http://www.nlm.nih.gov/medlineplus/encyclopedia.html. Accessed May 2008.
Quest Diagnostics. Thyroid Function Panel. Available online through http://www.questdiagnostics.com. Accessed May 2008.
Shomon, Mary. Thyroid Blood Tests. About.com Guide. Updated: March 27, 2007. Available online at http://thyroid.about.com/od/gettestedanddiagnosed/a/bloodtests.htm through http://thyroid.about.com. 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.
ARUP Lab Tests. Thyroid Panel. Available online at http://www.aruplab.com/guides/ug/tests/0070141.jsp through http://www.aruplab.com. Accessed February 2011.
National Endocrine and Metabolic Diseases Information Service. Thyroid Function Tests. Available online at http://endocrine.niddk.nih.gov/pubs/thyroidtests/index.htm through http://endocrine.niddk.nih.gov. Accessed February 2011.
MedlinePlus Medical Encyclopedia. TSH Test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003684.htm. Accessed February 2011.
MedlinePlus Medical Encyclopedia. T3 Test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003687.htm. Accessed February 2011.
MedlinePlus Medical Encyclopedia. T4 Test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003517.htm. Accessed February 2011.
MedlinePlus Medical Encyclopedia. T3RU Test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003688.htm. Accessed February 2011.
Mosby's Manual of Diagnostic and Laboratory Tests. Pagana and Pagana. 4th edition, Pg. 512.