What is being tested?
The thyroid gland is composed mainly of very small, ball-shaped structures called follicles. Thyroid follicle cells produce and store the thyroid hormones T4 or thyroxine and T3 or triiodothyronine as part of a protein called thyroglobulin, which occupies most of the centre of a follicle. When needed, the thyroglobulin is broken down to release thyroid hormones. No other part of the body makes this protein. Many thyroid cancers release small amounts of thyroglobulin into the bloodstream and this is what is measured by this test.
How is it used?
The main use of the thyroglobulin test is 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.
Based on the results of a thyroglobulin test, your 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. Your thyroglobulin levels will be checked again in a few weeks or months to verify that the therapy has worked.
When is it requested?
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.
What does the result mean?
Small amounts of thyroglobulin are normal in those with normal thyroid function.
Thyroglobulin levels should be undetectable or very low 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 your body.
If the level is low a few weeks or months after surgery and then begins to rise over time, the cancer may have returned.
Is there anything else I should know?
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.
Some thyroid cancer patients have antibodies to thyroglobulin called thyroglobulin antibodies or thyroglobulin autoantibodies. These antibodies can interfere with thyroglobulin testing, leading to falsely low or high results depending on the method used. For this reason, it is important that laboratories measure anti-thyroglobulin antibodies every time thyroglobulin is measured.
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 human TSH (rhTSH) is now available as an alternative. It is used to directly stimulate thyroglobulin production. Your doctor may request thyroglobulin measurement a few days after giving you an injection of rhTSH. Newer highly sensitive assays may remove the need for TSH stimulation.
Common questions
Thyroglobulin levels are not routinely measured before cancer treatment because the protein is produced by normal thyroid tissue and will be detectable in normal patients. An elevation does not mean that you have cancer and it is not necessarily closely related to the amount of cancer present.
More information
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