The thyroglobulin test is primarily used as a tumour marker to determine the effectiveness of thyroid cancer treatment and to monitor for recurrence. Since thyroglobulin normally is made only in the thyroid, it should drop to very low or undetectable levels in patients who have had their thyroid completely removed as part of thyroid cancer treatment. When measured after the completion of treatment, Thyroglobulin can thus be used to evaluate the effectiveness of treatment and may be requested to monitor for recurrence of the cancer.  Several thyroglobulin levels may be requested over a period of time (serial samples) to look at the change in concentration.  The change in thyroglobulin levels often provides more information than a single value.

A thyroglobulin antibody (TgAb) test may be requested 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 may interfere with the thyroglobulin test.  If present, the thyroglobulin measurements are unreliable.

A thyroglobulin test may be used after the surgical removal of your thyroid gland for cancer so your doctor can check for any normal and/or cancerous thyroid tissue that may have been left behind. It is often checked on a regular basis, even if negative after surgery, to make sure that the tumour has not come back or spread.

Small amounts of thyroglobulin are normal in patients with normal thyroid function. 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.

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.