Labcorp and its Specialty Testing Group, a fully integrated portfolio of specialty and esoteric testing laboratories.
To help evaluate thyroid gland function and to help diagnose thyroid disorders; to monitor treatment of thyroid disorders
When you have signs and symptoms suggesting underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) due to a condition affecting the thyroid; when you have an enlarged thyroid (goiter) or a thyroid nodule (a small lump on the thyroid gland that may be solid or a fluid-filled cyst)
A blood sample drawn from a vein
No test preparation is needed. However, certain medications, multivitamins and supplements can interfere with thyroid testing, so tell your healthcare practitioner about any prescribed or over-the-counter drugs and/or supplements that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day. Acute illness may affect thyroid hormone test results. It is generally recommended that thyroid testing be avoided in hospitalized patients or postponed until after you have recovered from the illness.
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 level of thyroid hormones in the blood.
A thyroid panel usually includes tests for:
Although rarely used these days, sometimes a T3 resin uptake (T3RU) test is included. T3RU and T4 can be used to calculate a free thyroxine index (FTI). This is another method for evaluating thyroid function. It corrects for changes in certain proteins that can affect total T4 levels.
The thyroid is a small butterfly-shaped organ that lies flat across the windpipe at the base of the neck that produces thyroid hormones, primarily T4 and some T3. These hormones travel throughout the body and regulate the metabolism by telling the cells in the body how fast to use energy and produce proteins. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.
Almost all of the T3 and T4 circulating in the blood is bound to protein. The small portions that are not bound are called "free" and are the biologically active forms of the hormones. Lab 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 body has an elaborate feedback system to control the amount of T4 and T3 in the blood.
Under normal circumstances, this feedback system regulates thyroid activity to maintain relatively stable levels of thyroid hormones in the blood.
If your thyroid gland does not produce enough T4 and T3 (underactive thyroid), due to thyroid dysfunction or to insufficient TSH, then you may have signs and symptoms of hypothyroidism such as weight gain, dry skin, cold intolerance, irregular menstruation, and fatigue. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. (See the article on Hashimoto Thyroiditis for more details.)
If your thyroid gland produces too much T4 and T3, you may have signs and symptoms associated with overactive thyroid (hyperthyroidism), such as rapid heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes and in some cases, bulging eyes. Graves disease is the most common cause of hyperthyroidism. (See the article on Graves Disease for more details.)
Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH.
A thyroid panel may be used to evaluate thyroid function and/or help diagnose thyroid disorders.
Typically, the preferred initial test for thyroid disorders is a TSH test. If the TSH level is abnormal, it will usually be followed up with a test for free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically. This is known as reflex testing and it saves the healthcare practitioner time from having to wait for the results of the initial test and then requesting additional testing to confirm or clarify a diagnosis. Reflex tests are typically performed on the original sample that was submitted when the initial test was requested.
As an alternative, a thyroid panel may be requested by your healthcare practitioner. This means that all three tests will be performed at the same time to get a more complete initial picture of thyroid function.
A thyroid panel may be ordered when you have signs and symptoms that suggest underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) due to a thyroid disorder.
Signs and symptoms of an underactive thyroid may include:
Signs and symptoms of an overactive thyroid may include:
If the feedback system involving the thyroid gland is not functioning properly due to one of a variety of disorders, then increased or decreased amounts of thyroid hormones may result.
The test results alone are not diagnostic but will prompt your healthcare practitioner to perform additional testing to investigate the cause of the excess or deficiency and thyroid disorder. As examples, the most common cause of hyperthyroidism is Graves disease and the most common cause of hypothyroidism is Hashimoto thyroiditis. (See the condition article on Thyroid Diseases for more on these and other related diseases.)
The following table summarizes some examples of typical test results and their potential meaning.
|Note: Laboratory results must always be correlated with the clinical findings of the patient.|
|TSH||Free T4||Total or Free T3||Most likely diagnosis|
|Normal||Normal||Normal||Normal thyroid function (e.g., "euthyroid")|
|Normal or decreased||Normal or decreased||Decreased||Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome)|
|Increased||Normal||Normal||Subclinical hypothyroidism1; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given|
|Increased||Decreased||Normal of decreased||Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism)|
|Normal or increased||Increased||Increased||Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance)|
|Decreased||Normal||Normal||Subclinical hyperthyroidism2; in a person with hypothyroidism, too much thyroid hormone is being given|
|Decreased||Normal||Increased||Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis)|
|Decreased||Increased||Increased||Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism)|
|Decreased||Decreased||Decreased||Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)|
1In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.
2In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.
The most common causes of thyroid dysfunction are autoimmune diseases. Graves disease causes hyperthyroidism and Hashimoto thyroiditis causes hypothyroidism. Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and too much or too little TSH.
Many multivitamins, supplements (especially hair and nail), and over-the-counter and prescription medications may affect thyroid test results and their use should be discussed with your healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for a thyroid panel.
Blood tests that may be performed in addition to a thyroid panel may include:
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when T4 is converted to T3 in the body, a certain percentage of the T3 is in the form of RT3. When the body is under stress, such as during a serious illness, thyroid hormone levels may be outside of normal ranges even though there is no thyroid disease present. RT3 may be elevated in non-thyroidal conditions, particularly the stress of illness. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness. Use of the RT3 test remains controversial, and it is not widely requested.
In the past, panels of tests were more common. More recently, however, the practice has been to order, where possible, one initial or screening test and then follow up with additional testing, if needed, to reduce the number of unnecessary tests. With thyroid testing, one strategy is to screen with a TSH test and then order additional tests if the results are abnormal or if clinical suspicions warrant.
Your thyroid hormone test results can be affected by:
Sources Used in Current Review
(2017 May. Updated). Thyroid Tests. National Institute of Diabetes and Digestive and Kidney Diseases. Available online at https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid. Accessed July 2020.
(2014). Thyroid Function Tests. American Thyroid Association. Available online at https://www.thyroid.org/wp-content/uploads/patients/brochures/FunctionTests_brochure.pdf. Accessed July 2020.
(July 2017, Updated). Hyperthyroidism. Home Health Network from the Endocrine Society. Available online at https://www.hormone.org/diseases-and-conditions/thyroid/hyperthyroidism. Accessed July 2020.
(March 2020, Updated).Straseski, J. Thyroid Disease. ARUP Consult. Available online at https://arupconsult.com/content/thyroid-disease. Accessed July 2020.
(2015 November 1, Updated). Sofronescu, A. Thyroid Screen Interpretation. Medscape Protocols. Available online at https://emedicine.medscape.com/article/2172202-overview. Accessed July 2020.
(2016 June 9). Dietrich, J. et. al. Calculated Parameters of Thyroid Homeostasis: Emerging Tools for Differential Diagnosis and Clinical Research. Front Endocrinol (Lausanne). 2016; 7: 57. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4899439/ Accessed July 2020.
(1995–2020). Thyroid Function Cascade, Serum. Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Overview/83633. Accessed July 2020.
(2020). ARUP Laboratories. Thyroid Panel. Available online at https://ltd.aruplab.com/Tests/Pub/0070141. Accessed July 2020.
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. 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. 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. 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. 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.
(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. 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/. 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. 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. Accessed January 2015.
Clarke, W., Editor (© 2011). Contemporary Practice in Clinical Chemistry 2nd Edition: AACC Press, Washington, DC. Pp 431-449.