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; it may be ordered in conjunction with other thyroid tests for certain thyroid disorders.
A blood sample drawn from a vein in your arm
Usually none; however, depending on the laboratory method used for measurement of thyroglobulin, some dietary supplements such as multivitamins or supplements rich in vitamin B7 (Biotin) should not be taken at least 12 hours before the test is performed in order to avoid any test interference that may lead to false results.
Based on information provided by the lab, your healthcare practitioner should inform you if this or any other preparation is necessary.
Thyroglobulin (Tg) is a protein produced by the thyroid gland. This test measures the amount of thyroglobulin in the blood.
The thyroid gland produces hormones (thyroxine or T4 and triiodothyronine or T3) that help 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 that produce and store thyroglobulin.
Thyroglobulin is produced all healthy individuals, and normally its concentration in blood is very low. Thyroglobulin concentration increases in both benign (Graves disease, subacute thyroiditis, Hashimoto thyroiditis) and malignant conditions (thyroid adenoma, thyroid papillary and follicular cancer). Therefore, it is on one hand a non-specific indicator of a thyroid dysfunction and, on the other hand, a tumor marker that can be used to monitor patients with thyroid papillary and follicular cancer after diagnosis (see Thyroid Cancer).
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
Usually no preparation is needed. However, depending on the laboratory method used for measurement of thyroglobulin, some dietary supplements such as multivitamins or supplements rich in vitamin B7 (Biotin) should not be taken at least 12 hours before the test is performed in order to avoid any test interference that may lead to false results.
Based on information provided by the lab, the healthcare practitioner should inform you if this or any other preparation is necessary.
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, the well-differentiated papillary and follicular thyroid cancers, frequently do, resulting in increased levels of thyroglobulin in the blood.
Thyroglobulin testing may be used, 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.
When is it ordered?
A thyroglobulin test may be ordered prior to the surgical removal of the thyroid gland for cancer. It is also ordered after the completion of treatment to help determine whether any normal and/or cancerous residual 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 (monitor for recurrence and/or metastasis).
A thyroglobulin test may sometimes be ordered when a person has symptoms of hyperthyroidism and/or an enlarged thyroid gland and the healthcare practitioner 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?
Thyroglobulin is present in the blood of all healthy individuals at very low concentration.
Thyroglobulin levels should be undetectable or very low after the surgical removal of the thyroid (thyroidectomy) and/or after subsequent radioactive iodine treatments. However, if upon monitoring post-surgery the thyroglobulin concentration in the person's blood is still detectable, there may still be some normal or cancerous residual thyroid tissue in the person's body, indicating the need for additional treatment.
Based on the results of a thyroglobulin test, a healthcare practitioner 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 and monitored periodically there afterwards.
If the level of thyroglobulin is low for a few weeks or months after surgery but then begins to rise over time, then the cancer is probably recurring or spreading (metastasis).
Decreasing levels of thyroglobulin in those treated for Graves disease indicate a response to treatment.
Is there anything else I should know?
Increased concentrations of thyroglobulin are not diagnostic of thyroid cancer. Cancer must be diagnosed by looking at samples of 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 (pattern) is more important than one particular thyroglobulin test result.
Laboratories may use different methods to test for thyroglobulin, so results can vary from lab to lab. If you are having a series of thyroglobulin tests done (several tests done on different days), it is advised that you have the tests done by the same method, typically by the same laboratory, so that the results can be compared and interpreted correctly. You may wish to discuss this issue with your healthcare provider.
Thyroglobulin antibodies (TgAb) are immune proteins produced that target thyroglobulin. Depending on the test method used by the laboratory, these antibodies can interfere with the results of the thyroglobulin test if they are present in the blood of the person being tested. If the antibodies are present, either the Tg test cannot be used to monitor levels or a method that is not affected by TgAb must be used to monitor levels. Read the article on Thyroid Antibodies to learn more.
Those who have their thyroid removed will need to take thyroid hormone replacement (thyroxine) for the rest of their life. In the past, a healthcare practitioner may have had someone on thyroxine refrain from taking it for up to several weeks prior to thyroglobulin testing. This is because low thyroxine will stimulate the production of TSH and, in turn, this will stimulate the production of thyroglobulin by any remaining normal or cancerous thyroid tissue. Therefore, cessation of thyroxin therapy will make the thyroglobulin test more sensitive, but it often leaves the person being tested with uncomfortable hypothyroid symptoms until testing is completed. A recombinant form of TSH is now available as an alternative. It is used to directly stimulate thyroglobulin production.
Should everyone have a thyroglobulin test?
Why didn't my doctor check my thyroglobulin level before removing my thyroid?
Your thyroglobulin (Tg) level may or may not be measured before your thyroid gland is removed as part of the treatment for thyroid cancer. Since the protein is produced by normal thyroid tissue as well as by many thyroid cancers and can be elevated with many thyroid conditions, it can be difficult to determine whether an increased thyroglobulin is due to the cancer.
Is there anything I can do to raise or lower my thyroglobulin level?
Can thyroglobulin testing be performed in my healthcare practitioner's office?
No, it requires specialized equipment to perform. The test may not be offered in every laboratory and may need to be sent to a reference laboratory. Due to certain technical limitations of the assay, normal reference ranges can vary greatly. Thus, when serial thyroglobulin levels are required, they should be performed at the same laboratory.