Thyroglobulin

What is being tested?
This test measures the concentration of thyroglobulin in the blood. Thyroglobulin is a protein produced by the thyroid, the gland that regulates the body’s metabolism. The thyroid is a small butterfly-shaped organ in the neck, lying against the windpipe, and is made up of small spherical structures called follicles. The follicle cells produce and store thyroglobulin, which, when needed, is broken down to form the thyroid hormones T4 (thyroxine) and T3 (triiodothyronine). The production of these hormones and their release into the bloodstream are stimulated by the pituitary hormone TSH (thyroid-stimulating hormone). Thyroglobulin is not made by any other organ, but it is produced by many thyroid cancers, whether confined to the gland or spread to other parts of the body.

How is the sample collected for testing?
The blood sample is obtained by drawing blood from a vein
.
Is any preparation needed to ensure the quality of the sample?
No preparation is needed.

1. How is it used?
2. When is it requested?
3. What does the test result mean?
4. What else should I know?

1. How is it used?
Thyroglobulin is used mainly as a tumour marker to assess the effectiveness of treatment for thyroid cancer and to monitor for recurrence. Not all thyroid cancers produce thyroglobulin, but the most common ones, such as papillary and follicular thyroid cancer, do, causing blood levels to rise. Thyroglobulin may be requested together with TSH before thyroid cancer treatment begins, to find out whether the tumour is producing it. Thyroglobulin levels can then be measured repeatedly (serial samples) to see whether they change over time. The change in concentration often gives more information than a single value.
Anti-thyroglobulin antibodies (TgAb) may be requested together with thyroglobulin. These are proteins produced by the immune system against thyroglobulin. These antibodies can be produced at any time. When present, they bind to circulating thyroglobulin molecules and interfere with the interpretation of the test. Once produced, they persist and reduce the usefulness of the thyroglobulin test.

2. When is it requested?
The thyroglobulin test may be requested before surgical removal of the thyroid for cancer. It may also be prescribed at the end of surgery to see whether any normal and/or cancerous thyroid tissue remains. It is often prescribed at regular intervals after surgery to detect any recurrence or spread of the tumour.
The thyroglobulin test may also be prescribed when there are symptoms of hyperthyroidism and/or an enlarged thyroid, if thyroid disorders such as Graves’ disease or thyroiditis are suspected. It may be prescribed at intervals during treatment with antithyroid drugs (in conditions such as Graves’ disease) to assess the effectiveness of treatment.
Rarely, it may be prescribed when there are symptoms of hypothyroidism in childhood.

3. What does the test result mean?
Low levels of thyroglobulin are normally present when the thyroid is working normally. If thyroglobulin levels are initially high in patients with thyroid cancer, it is likely that it can be used as a tumour marker. Thyroglobulin levels should be undetectable or very low after thyroidectomy and/or radioactive iodine treatment. If levels are still detectable, normal or cancerous thyroid tissue remains, suggesting that further treatment may be needed.
Depending on the thyroglobulin results, a thyroid scan or radioactive iodine treatment may be needed to find and/or destroy any remaining normal or cancerous thyroid tissue. Thyroglobulin levels should then be checked again within a few weeks or months to assess the effect of treatment.
If thyroglobulin levels stay low for some weeks or months after thyroidectomy and then start to rise, the cancer has probably come back.
If anti-thyroglobulin antibodies are present, the thyroglobulin test may be falsely high or low. Results must be interpreted with caution. The presence of anti-thyroglobulin autoantibodies reduces or limits the usefulness of the thyroglobulin test as a tumour marker.
A fall in thyroglobulin levels in treated patients with Graves’ disease indicates a response to treatment. With goitre, thyroiditis or hyperthyroidism, thyroglobulin levels may be high, although they are not routinely requested.

4. What else should I know?
Raised thyroglobulin levels do not diagnose thyroid cancer. The diagnosis is made by histological examination.
High thyroglobulin levels do not in themselves indicate a poor prognosis. When monitoring for recurrence, the change in levels over time is more important than a single result. It is important to have serial results from the same laboratory, because different laboratories may produce different results.
In 15-20% of thyroid cancer cases, anti-thyroglobulin antibodies (also called anti-thyroglobulin autoantibodies) are produced.
After thyroidectomy, lifelong thyroxine replacement therapy is needed. In the past, it was stopped for a few weeks before thyroglobulin was measured. This stimulated TSH production and the production of thyroglobulin by any remaining normal or cancerous thyroid tissue, increasing the sensitivity of the thyroglobulin test but causing symptoms of hypothyroidism. A recombinant TSH is now available that directly stimulates thyroglobulin production

1. Why isn’t thyroglobulin measured before thyroidectomy?
2. Can the development of anti-thyroglobulin antibodies be prevented?
3. Can anyone have the thyroglobulin test?
4. Is there anything that can raise or lower thyroglobulin levels?
5. Can the thyroglobulin test be done at my GP’s surgery?

1. Why isn’t thyroglobulin measured before thyroidectomy?
Thyroglobulin may or may not be measured before thyroidectomy for thyroid cancer. Because the protein is produced by both normal and cancerous thyroid tissue and can be raised in many thyroid conditions, it is not possible to tell whether high thyroglobulin levels are due to cancer.

2. Can the development of anti-thyroglobulin antibodies be prevented?
No, they can appear at any time regardless of lifestyle changes. This is why it is important to test for them during monitoring for recurrence.

3. Can anyone have the thyroglobulin test?
It is not a screening test for the general population. It is not specific for thyroid cancer, and thyroglobulin can be temporarily raised in many thyroid conditions.

4. Is there anything that can raise or lower thyroglobulin levels?
Not directly. Its concentration reflects the presence of normal or cancerous thyroid tissue and does not depend on lifestyle changes.

5. Can the thyroglobulin test be done at my GP’s surgery?
No, it requires special equipment. The test is not carried out in every laboratory, and the sample may need to be sent to a reference laboratory