What is a thyroglobulin test?

A thyroglobulin test measures the level of thyroglobulin in a sample of your blood. Thyroglobulin is a protein that your thyroid makes. Your thyroid is a small, butterfly-shaped gland in your neck. It makes hormones that control many activities in your body, including your heart rate and how fast you burn calories from food.

A thyroglobulin test is a type of tumor marker test. Tumor markers are substances made by cancer cells and/or by normal cells in response to cancer in your body. Normally, your thyroid releases small amounts of thyroglobulin into your bloodstream. Cells from common types of thyroid cancer (papillary carcinoma and follicular thyroid cancer) also release thyroglobulin.

Thyroglobulin testing is not used to diagnose thyroid cancer because other thyroid diseases that aren't cancer can also affect thyroglobulin levels. But the test is useful after treatment for common thyroid cancers to see if the treatment worked. That's because the goal of treatment is to get rid of all thyroid tissue – both healthy cells and cancer cells. So, if treatment is successful, there should be little or no thyroglobulin in your blood. If thyroglobulin levels remain the same or increase, more cancer treatment may be needed.

Other names: Tg, TGB. thyroglobulin tumor marker

What is it used for?

A thyroglobulin test is mostly used to:

• See if thyroid cancer treatment was successful and guide decisions about more treatment.
• Predict how your cancer will behave over time.
• See if cancer has returned after successful treatment.

Thyroglobulin tests may also be used to help diagnose hyperthyroidism and hypothyroidism, which are common thyroid conditions that aren't cancer.

Why do I need a thyroglobulin test?

You may need a thyroglobulin test before starting treatment for thyroid cancer. Your health care provider will compare these test results to your test results after treatment.

You may also need this test a few weeks after you finish treatment for thyroid cancer. The test helps show whether any thyroid cells remain in your body. If your treatment was successful, you may still need to have your thyroglobulin levels tested from time to time to see if cancer has come back.

What happens during a thyroglobulin test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You usually don't need any special preparations for a thyroglobulin test. But you need to avoid taking certain vitamins or supplements so tell your provider about everything you take.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Understanding the results of a thyroglobulin test after treatment can be complicated. The meaning of your results depends on your health history, the type of treatment you had, and the results of other tests. So, it's best to ask your provider to explain what your test results say about your health.

In general, if you were tested after treatment for thyroid cancer:

• Very low levels or no thyroglobulin may mean that your cancer treatment has worked to get rid of all thyroid tissue, including cancer. But you will still need more testing over time to make sure.
• Thyroglobulin levels that stay high or increase may mean that:
  • Your treatment did not get rid of all thyroid tissue in your body.
  • You still have thyroid cancer in your body that has grown and may have spread.
• Thyroglobulin levels that were low after treatment but later increased may mean that your thyroid cancer has come back after treatment.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a thyroglobulin test?

Labs use different methods to measure the amount of thyroglobulin in your blood sample. The test method can affect your results. So, it's important to have your tests done the same way, and usually in the same lab. This allows your provider to compare your results over time. If you have questions about this, talk with your provider.

A thyroglobulin test may not be useful for monitoring your thyroid cancer treatment if you have thyroglobulin antibodies in your blood. These antibodies are proteins that your immune system may make. They attach to thyroglobulin and can make your thyroglobulin levels appear lower than they really are.

Your provider will usually order a thyroid antibody test to find out whether thyroglobulin testing will be useful. If you have thyroglobulin antibodies, other tests will be used to see if your cancer treatment was successful.