Once treatment for thyroid cancer has been completed, before and after radioactive iodine therapy for thyroid cancer, and at varying intervals to monitor for recurrence.
A blood sample taken from a vein in your arm
No fasting or special preparation is required before the test. In order to increase the ability of the test to pick up very small amounts of remaining thyroid cells some patients may be asked to stop taking their thyroid hormone replacement tablets prior to the test or be given injections (recombinant TSH) in an attempt to stimulate thyroglobulin production. Please follow any instructions you are given by your Doctor, prior to having this blood test.
The thyroid gland is a small butterfly shaped organ that helps to regulate the rate at which the body uses energy. The thyroid lies flat against the windpipe in the throat and is composed primarily of very small, ball-shaped structures called follicles. This test measures the amount of thyroglobulin in the blood.
Thyroglobulin is a protein produced by follicle cells and stored in the thyroid gland. Thyroglobulin is broken down when needed into the thyroid hormones T4 (thyroxine) and T3 (triidothyronine). The production of these hormones and their release into the blood stream is stimulated by the hormone TSH (thyroid stimulating hormone). No other part of the body makes thyroglobulin, but it is produced by many thyroid cancers – both those confined to the thyroid gland and those that have spread to other parts of the body.
How is the sample collected for testing?
A blood sample is usually obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed, but please follow any instructions you are given by your Doctor, prior to having this blood test.
How is it used?
The thyroglobulin blood 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 blood tests may be performed over a period of time (serial samples) to look at the change in concentration. The change in thyroglobulin results often provides more information than a single value.
A thyroglobulin antibody (TgAb) test may be requested at the same time as 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 affect the thyroglobulin test results. If present, the thyroglobulin measurements are unreliable.
When is it requested?
A thyroglobulin test may be requested 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 test result mean?
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. The production of thyroglobulin by any remaining cells can be increased by stopping thyroid hormone replacement treatment or by giving injections of TSH, improving the chances of detecting small numbers of cells. This technique is used only in some cases and is dependent on factors like the size and type of cancer.
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 check that the treatment has worked.
If the level of thyroglobulin is low for a few weeks or months after surgery and then begins to rise over time, it is possible the cancer has returned.
Is there anything else I should know?
Elevated levels of thyroglobulin do not in themselves imply a bad outcome. 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 (1 in 5) have thyroglobulin antibodies (also called thyroglobulin autoantibodies). These antibodies can lead to falsely high or low thyroglobulin results depending on the method used.
Why didn't my doctor check my thyroglobulin level before removing my thyroid?
Thyroglobulin levels are not normally measured before thyroid cancer treatment because it is produced by normal thyroid tissue as well. Thyroglobulin can also be elevated in many other thyroid conditions. A high level does not mean that you have cancer, nor does the level indicate how much cancer is present.