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, 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.

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.

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.

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.