Thyroglobulin (TG)

What is thyroglobulin?

Thyroglobulin is a large protein produced only by the cells of the thyroid gland. The hormones T4 and T3 are formed on the structure of this protein and stored within the gland. Normally, a small amount of thyroglobulin passes into the blood; as the amount of thyroid tissue increases or the gland is stimulated, its level in the blood also rises. No tissue in the body other than the thyroid produces this protein, which is also why the test is useful in cancer follow-up.

The test is performed on a blood sample using an immunological method. Antibodies against thyroglobulin (Anti-TG) in the blood can interfere with the measurement and make the result appear lower than it really is; for this reason, thyroglobulin is usually ordered together with Anti-TG. Because methods differ from one another, it is recommended that follow-up measurements be done at the same laboratory.

When is the thyroglobulin test ordered?

The most important use of thyroglobulin is post-treatment monitoring of papillary and follicular thyroid cancers. In people whose thyroid gland has been surgically removed and who have received radioactive iodine treatment if needed, thyroglobulin is expected to be very low; a rise over time is investigated for the possibility that the disease has recurred. This test is not used to diagnose or screen for thyroid cancer in people whose gland is still in place. Your doctor decides how often checks are done and which method is used, based on the risk status of the disease.

• Regular follow-up after thyroid cancer surgery and radioactive iodine treatment
• Assessment of response to treatment
• Further investigation when recurrence of the disease is suspected
• Stimulated thyroglobulin measurement planned by the doctor
• Identifying external hormone intake in hyperthyroidism of unclear cause

How do you prepare for the thyroglobulin test?

No fasting is required for the test. Because thyroglobulin can rise temporarily after a fine-needle biopsy of the thyroid, surgery or radioactive iodine treatment, your doctor schedules the test for a few weeks after these procedures. If a stimulated measurement is planned, your doctor determines the preparation steps, such as injections or pausing medication, as well as the day of the blood draw. Keeping your previous thyroglobulin results makes it easier to compare values.

Supplements containing biotin (vitamin B7) can make thyroglobulin appear falsely low with some measurement methods; because this can be misleading during follow-up, be sure to mention it if you take them. You can take your thyroid hormone medication as usual unless your doctor tells you otherwise. For this test, which requires regular follow-up, blood can be drawn by a nurse at your home or workplace.

What does the thyroglobulin result mean?

In people whose thyroid gland has been completely removed and whose treatment is complete, thyroglobulin that is too low to measure is generally a favorable finding. A measurable level, or a rise over consecutive measurements, may indicate remaining thyroid tissue or recurrence of the disease and may call for further investigations such as imaging. In follow-up, the trend of the values over time matters more than any single value. In people who are Anti-TG positive, a low thyroglobulin result on its own is not considered reassuring.

In people whose thyroid gland is still in place, high thyroglobulin can also be seen in goiter, nodules, Graves' disease or thyroid inflammation, and it does not mean cancer. The laboratory's reference range on this page is generally set for people whose thyroid gland is in place; your doctor determines the target value after surgery. Ranges can also differ by laboratory, age and sex. Your result should be interpreted by your doctor together with your Anti-TG level, imaging findings, symptoms and medical history.