Thyroglobulin test explained

Thyroglobulin (Tg) is a protein produced by the thyroid gland. The thyroglobulin blood test measures the amount in a sample taken from a vein in the arm. Small amounts are present in people with normal thyroid function, according to Lab Tests Online UK.

This is why the test is not normally used to diagnose thyroid cancer. Thyroglobulin is produced by normal thyroid tissue as well as by cancerous thyroid cells, and it can be raised in many other thyroid conditions. The test becomes useful when the thyroid has been removed or treated, because the expected level is then very low.

The NHS says surgery is the most common treatment for thyroid cancer, and that depending on the type and stage you may have all of the thyroid removed (total thyroidectomy) or part of it. Radioactive iodine treatment may follow surgery to kill any cancer cells that may have been left behind, or to help stop the cancer coming back.

After treatment, thyroglobulin acts as a marker. Lab Tests Online UK explains that it should drop to very low or undetectable concentrations in people whose thyroid has been completely removed, so a detectable level can point to remaining normal thyroid tissue or cancerous tissue, and a level that is low at first and then begins to rise over time may suggest the cancer has returned.

It is mainly used for the follow-up of the types of thyroid cancer that arise from the cells that make thyroglobulin. The British Thyroid Foundation describes thyroglobulin as a more specialised test used in monitoring people who have already been treated for differentiated thyroid cancer, and mentions calcitonin as the equivalent for medullary thyroid cancer.

Lab Tests Online UK lists the usual times as once treatment is complete, before and after radioactive iodine therapy, and at varying intervals afterwards to monitor for recurrence. It is often checked regularly even if negative after surgery, to make sure the tumour has not come back or spread.

• After surgery to check for remaining thyroid tissue.

• Before and after radioactive iodine treatment.

• At intervals during long-term follow-up, even when earlier results were negative.

No fasting or special preparation is needed for the blood draw. However, Lab Tests Online UK says that to improve the chance of picking up very small amounts of remaining thyroid cells, some people may be asked to stop their thyroid hormone replacement tablets beforehand or be given injections of recombinant TSH to stimulate thyroglobulin production. This is used only in some cases and depends on factors such as the size and type of cancer. Follow your team's instructions exactly.

Biotin (vitamin B7) supplements should not be taken in the day before the test, because they can interfere with the assay and lead to false results.

Thyroglobulin should be undetectable or very low in everyone after thyroidectomy, and after radioactive iodine treatment. If concentrations are still detectable, Lab Tests Online UK says there may be normal or cancerous thyroid tissue remaining, indicating the need for additional treatment. A doctor may follow up with a radioactive iodine scan or further treatment, and levels are then checked again after a few weeks or months.

The same source is careful to say that elevated concentrations do not in themselves imply a bad outcome. Interpretation depends on whether the thyroid was removed completely, whether radioactive iodine was given, whether thyroid hormone tablets were stopped or TSH stimulation used, and on the trend between tests.

A thyroglobulin antibody (TgAb) test is often requested at the same time. These are proteins the immune system makes against thyroglobulin. Lab Tests Online UK says they can develop at any time and that, when present, thyroglobulin measurements are unreliable, with about 1 in 5 people with thyroid cancer affected.

The British Thyroid Foundation notes that in people without thyroid cancer, thyroglobulin antibodies are sometimes raised in Hashimoto's thyroiditis, and that repeating antibody tests in adults is rarely useful. In cancer follow-up the antibody result mainly tells the team how far to trust the thyroglobulin number, and whether they may need to follow the antibody level itself over time.

On the NHS, thyroglobulin monitoring is arranged by the endocrinology or cancer team looking after you, and the NHS says that you should contact your specialists if you have symptoms or side effects that worry you rather than waiting for the next check-up.

Some people also see a private endocrinologist or thyroid surgeon. Whichever route you use, the practical points are the same: ask which thyroglobulin method is used, keep the same laboratory for serial tests if possible, and ask how antibodies will be handled.