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 uniquely by your 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 at the base of the throat and is composed primarily of very small, ball-shaped structures called follicles that produce and store thyroglobulin.
Thyroglobulin is produced in all healthy individuals, and normally its level in blood is low. Thyroglobulin levels increase in both benign (Graves disease, subacute thyroiditis, Hashimoto thyroiditis) and thyroid 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 the Thyroid Cancer article).
How is the test 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 levels. The change or pattern of results often provides more information than a single value.
Rarely, the test may be ordered to help differentiate between subacute thyroiditis and thyrotoxicosis factitia (hyperthyroidism caused by ingesting excess thyroid hormone pills) and to determine the cause of congenital hypothyroidism in newborns.
When is it ordered?
A thyroglobulin test may be ordered prior to surgery to remove the thyroid gland for cancer. It is also 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 (monitor for recurrence and/or metastasis).
A thyroglobulin test may sometimes be ordered when you have symptoms of overactive thyroid (hyperthyroidism) and/or an enlarged thyroid gland and your healthcare practitioner suspects that you may have a thyroid disorder such as Graves disease or thyroiditis. It may be ordered at intervals when you are being treated with anti-thyroid medications (for conditions such as Graves disease) to determine how well the treatment is working.
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 a low concentration.
If you have thyroid cancer and your thyroglobulin level is elevated, then it is likely that thyroglobulin can be used as a tumor marker. In monitoring for cancer recurrence, the trend over time (pattern) is more important than one particular thyroglobulin test result.
- Thyroglobulin levels should be undetectable or very low after the surgical removal of the thyroid (thyroidectomy) and/or after subsequent radioactive iodine treatments, provided that you are taking your daily thyroid hormone medication (levothyroxine, T4) to prevent TSH from rising.
- However, if after surgery the thyroglobulin in your blood is still detectable, there still may be either some normal or cancerous residual thyroid tissue remaining, indicating the need for imaging scans or additional treatment.
- Based on the results of a thyroglobulin test, your healthcare practitioner may follow up with an ultrasound and/or 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 the effect of the therapy 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 while you are taking your daily dose of thyroid hormone, then the cancer is possibly recurring or spreading (metastasis), prompting your healthcare practitioner to order anatomic imaging or further treatment.
Decreasing levels of thyroglobulin in those treated for Graves disease indicate a response to treatment.
Is there anything else I should know?
Increased thyroglobulin levels are not diagnostic of thyroid cancer. Cancer must be diagnosed by looking at samples of cells (biopsy) using a microscope. Elevated levels of thyroglobulin do not in themselves imply a poor prognosis. For more details, see the articles on Thyroid Cancer and Thyroid Nodules.
If you have your thyroid removed, you will need to take thyroid hormone replacement (thyroxine) for the rest of your life.
I will have several thyroglobulin tests done over time. Why do I need to have them done at the same lab?
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.
I also had a test for thyroglobulin antibodies done. Why?
Thyroglobulin antibodies (TgAb) are immune proteins produced that target thyroglobulin. Approximately 25% of patients diagnosed with thyroid cancer have detectable TgAb. Depending on the test method used by the laboratory, these antibodies can interfere with the results of the thyroglobulin test when present in the blood of the person being tested. If the antibodies are present, the Tg test may be unreliable for monitoring thyroglobulin levels. Alternatively, a test method that is not affected by TgAb may be used to monitor thyroglobulin. Recently it has become recognized that monitoring the trend in the TgAb level itself can be a useful marker for the presence/or absence of disease. Read the article on Thyroid Antibodies to learn more.
Why didn't my healthcare practitioner 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. However, once a diagnosis of thyroid cancer has been made from a biopsy of a thyroid nodule, a high thyroglobulin level before surgery is a useful indicator that the tumor produces thyroglobulin, indicating the usefulness of thyroglobulin as a post-operative tumor marker.
Is there anything I can do to raise or lower my thyroglobulin level?
No, not directly. This test reflects the presence of normal and/or cancerous thyroid tissue and is not affected by lifestyle changes. Missing doses of your daily thyroid hormone treatment can cause TSH to rise, which in turn can stimulate thyroglobulin production from the small amount of normal thyroid tissue that typically remains following surgery for thyroid cancer.
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 methods, normal reference ranges can vary and some thyroglobulin tests are unreliable in the presence of thyroglobulin antibodies (TgAb). Thus, when serial thyroglobulin levels are required, they should be performed at the same laboratory.