Thyroglobulin (Tg)

Normally, your thyroid releases small amounts of thyroglobulin into your bloodstream. However, various conditions that affect the thyroid can lead to higher levels of this protein. The most common causes of high thyroglobulin levels are two types of thyroid cancer called papillary carcinoma and follicular thyroid cancer. In both conditions, cancerous cells produce thyroglobulin. Because of this, thyroglobulin is usually measured as a tumor marker. Tumor markers are substances made by cancer cells and/or by normal cells in response to cancer in your body.

Other thyroid diseases can also affect your thyroglobulin levels. Because such a wide range of conditions can affect thyroglobulin levels, health care providers don't use the results from a thyroglobulin test to diagnose cancer. Instead, the test is mainly used to follow people who have already been diagnosed with certain thyroid cancers. After surgery to remove the thyroid and follow-up treatment to destroy any remaining thyroid tissue, thyroglobulin should fall to very low levels, so a rising level can be a sign that thyroid cancer has come back or spread. Low thyroglobulin production can also be seen in some infants born with an underactive thyroid and goiter.

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.

After thyroid removal and follow-up treatment for thyroid cancer, very low Tg results (typically below about 0.1 to 0.2 ng/mL while taking thyroid hormone) are reassuring and suggest a low chance of remaining or returning cancer. Levels above about 1 ng/mL while not stimulated, or above 2 ng/mL after stimulation, are more concerning and usually lead to more testing such as neck ultrasound or imaging.

Because other thyroid conditions and certain illnesses can also affect thyroglobulin, your provider will likely check your levels over time instead of relying on a single test result.

If your blood also contains antibodies against thyroglobulin, they can interfere with the test and may make the thyroglobulin result look lower than it really is. Your provider may check for these antibodies along with thyroglobulin so the results are easier to interpret.