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) that arise from the follicle cells frequently do - resulting in increased levels of thyroglobulin in the blood. Thyroglobulin may be ordered, often along with a TSH test, prior to thyroid cancer treatment to determine whether the cancer is producing thyroglobulin. If it is, then it can be measured again after the completion of treatment to evaluate the effectiveness of treatment and may be ordered at intervals 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 may be ordered along with the thyroglobulin test. Thyroglobulin antibodies are proteins that the body’s immune system develops to attack thyroglobulin. These antibodies can develop at any time and when they are present, they interfere with the thyroglobulin test. Once they have developed, they will not go away and from that point forward, the TgAb test should be ordered with every 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 Grave’s disease.

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 thyroid tissue (normal and/or cancerous) 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 patient has symptoms of hyperthyroidism and/or an enlarged thyroid gland and the doctor suspects that the patient may have a thyroid disorder such as Grave’s disease or thyroiditis. It may be ordered at intervals when a patient is being treated with anti-thyroid medications (for conditions such as Grave’s disease), to determine the effectiveness of treatment.

Small amounts of thyroglobulin are normal in patients with normal thyroid function. Patients with a goiter, thyroiditis (inflamed thyroid), or hyperthyroidism may have elevated thyroglobulin levels, although the test is not routinely ordered with these conditions. If thyroglobulin concentrations are elevated in a patient with a thyroid cancer, then it is likely that thyroglobulin can be used as a tumor marker. Thyroglobulin levels should be undetectable or very low in all patients after a thyroidectomy (surgical removal of the thyroid) and/or after subsequent radioactive iodine treatments. If levels are still detectable, there may be normal or cancerous thyroid tissue remaining in the patient’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 (iodine is needed to make thyroid hormones) 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. Decreasing levels of thyroglobulin in patients treated for Grave’s disease indicate a response to treatment.

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 thyroid cancer patients have thyroglobulin antibodies (also called thyroglobulin autoantibodies). These antibodies can lead to falsely low or high thyroglobulin results depending on the method used.