At a Glance
Why Get Tested?
To monitor treatment of thyroid cancer and to detect recurrence; less commonly, to help determine the cause of hyperthyroidism and hypothyroidism
When to Get Tested?
Prior to and after the completion of treatment for thyroid cancer, before and after radioactive iodine therapy, and at regular intervals to monitor for recurrence
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
The Test Sample
What is being tested?
This test measures the amount of thyroglobulin in the blood. Thyroglobulin is a protein produced by the thyroid gland. The thyroid gland helps to regulate the rate at which the body uses energy. It is a small butterfly-shaped organ that lies flat against the windpipe in the throat and is composed primarily of very small, ball-shaped structures called follicles. Cells in the follicles produce and store thyroglobulin, breaking it down as needed into the thyroid hormones T4 (thyroxine) and T3 (triiodothyronine). The production of these hormones and their release into the blood stream is stimulated by the pituitary hormone TSH (thyroid stimulating hormone). No other part of the body makes thyroglobulin, but it is produced by many thyroid cancers, both those confined to the thyroid gland and those that have spread to other parts of the body.
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.
The Test
How is it used?
The thyroglobulin test is primarily used as a tumor marker to evaluate the effectiveness of treatment for thyroid cancer and to monitor for recurrence. Not every thyroid cancer will produce thyroglobulin, but the most common types, papillary and follicular thyroid cancer, frequently do, resulting in increased levels of thyroglobulin in the blood. Thyroglobulin may be ordered, along with a TSH test, prior to thyroid cancer treatment to determine whether the cancer is producing thyroglobulin. If it is, then the test can be ordered at intervals after treatment to monitor for cancer recurrence. Several thyroglobulin levels may be ordered over a period of time (serial samples) to look at the change in concentration. The change often provides more information than a single value.
A thyroglobulin antibody (TgAb) test is typically ordered along with the thyroglobulin test. Thyroglobulin antibodies are proteins produced by the body's immune system to attack thyroglobulin. These antibodies can develop at any time. When they are present, they bind to any thyroglobulin that may be present in the blood and interfere with the interpretation of the thyroglobulin test. Once they have developed, they will not go away and from that point forward will affect the usefulness of the thyroglobulin test.
Thyroglobulin testing is also occasionally ordered to help determine the cause of hyperthyroidism and to monitor the effectiveness of treatment for conditions such as Graves disease. Rarely it may be ordered to help determine the cause of congenital hypothyroidism in infants.
When is it ordered?
A thyroglobulin test may be ordered prior to the surgical removal of the thyroid gland for cancer. It may also be ordered after the completion of treatment to help determine whether any normal and/or cancerous thyroid tissue may have been left behind. It is often ordered on a regular basis after surgery to make sure that the tumor has not come back or spread.
A thyroglobulin test also may be ordered when a person has symptoms of hyperthyroidism and/or an enlarged thyroid gland and the doctor suspects that the person may have a thyroid disorder such as Graves disease or thyroiditis. It may be ordered at intervals when someone is being treated with anti-thyroid medications (for conditions such as Graves disease), to determine the effectiveness of treatment.
Rarely it may be ordered when an infant has symptoms associated with hypothyroidism.
What does the test result mean?
Small amounts of thyroglobulin are normal in those with normal thyroid function. If thyroglobulin concentrations are initially elevated in a person with thyroid cancer, then it is likely that thyroglobulin can be used as a tumor marker. Thyroglobulin levels should be undetectable or very low after the surgical removal of the thyroid (thyroidectomy) and/or after subsequent radioactive iodine treatments. If levels are still detectable, there may be normal or cancerous thyroid tissue remaining in the person's body, indicating the need for additional treatment.
Based on the results of a thyroglobulin test, a doctor may follow up with a radioactive iodine scan and/or radioactive iodine treatments to identify and/or destroy any remaining normal thyroid tissue or thyroid cancer. Thyroglobulin levels are then checked again in a few weeks or months to verify that the therapy has worked.
If the level of thyroglobulin is low for a few weeks or months after surgery and then begins to rise over time, then the cancer is probably recurring.
If a person develops thyroglobulin antibodies, then the thyroglobulin test results may be falsely elevated or decreased, depending on the testing method used. Results must be interpreted with caution. The presence of the antibodies lessens or eliminates the usefulness of the thyroglobulin test as a tumor marker.
Decreasing levels of thyroglobulin in those treated for Graves disease indicate a response to treatment.
People who have a goiter, thyroiditis, or hyperthyroidism may have elevated thyroglobulin levels, although the test is not routinely ordered with these conditions.
Is there anything else I should know?
Increased concentrations of thyroglobulin are not diagnostic of thyroid cancer. Cancer must be diagnosed by looking at sampled cells (biopsy) under a microscope. Elevated levels of thyroglobulin do not in themselves imply a poor prognosis. In monitoring for cancer recurrence, change over time is more important than one particular thyroglobulin test result.
It is important to have serial thyroglobulin tests performed at the same laboratory because test methods may produce different results in different laboratories.
Fifteen to twenty percent of people with thyroid cancer develop thyroglobulin antibodies (also called thyroglobulin autoantibodies).
Those who have their thyroid removed will need to take thyroid hormone replacement (thyroxine) for the rest of their life. In the past, a doctor may have had someone on thyroxine refrain from taking it for up to several weeks prior to thyroglobulin testing. This stimulated the production of TSH and the production of thyroglobulin by any remaining normal or cancerous thyroid tissue. It made the thyroglobulin test more sensitive, but it often left the person being tested with uncomfortable hypothyroid symptoms. A recombinant form of TSH is now available as an alternative. It is used to directly stimulate thyroglobulin production.
Common Questions
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Article Sources
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
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 916-918.
Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 372.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1030-1031.
(Revised 2009 May 14). How Is Thyroid Cancer Diagnosed? American Cancer Society [On-line information]. Available online at http://www.cancer.org/docroot/cri/content/cri_2_4_3x_how_is_thyroid_cancer_diagnosed_43.asp?sitearea=&level= through http://www.cancer.org. Accessed October 2009.
Grebe, S. (2009 March 25). Diagnosis and Management of Thyroid Carcinoma: A Focus on Serum Thyroglobulin. Medscape Today from Expert Review of Endocrinology and Metabolism [On-line information]. Available online at http://www.medscape.com/viewarticle/586810 through http://www.medscape.com. Accessed October 2009.
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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.
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Whitley, RJ and Ain, KB (2004). Thyroglobulin: a specific serum marker for the management of thyroid carcinoma. Medscape from Clin Lab Med. 2004; 24(1):29-47 (ISSN: 0272-2712) [On-line Abstract] Available online at http://www.medscape.com/medline/abstract/15157556?queryText=thyroglobulin through http://www.medscape.com.
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(© 2006). Thyroglobulin and Thyroglobulin Antibody. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.arup-lab.com/guides/clt/tests/clt_249b.jsp#1149079 through http://www.arup-lab.com.
