Thyroglobulin

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.

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, the test may be ordered to help differentiate between subacute thyroiditis and thyrotoxicosis factitia and to determine the cause of congenital hypothyroidism in newborns.

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.

Thyroglobulin is present in the blood of all healthy individuals at very low concentration.

If thyroglobulin concentrations are initially elevated in a person diagnosed 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. 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.

People who have a goiter, thyroiditis, or hyperthyroidism may have elevated thyroglobulin levels, although the test is not routinely ordered with these conditions.

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.