What Are Thyroglobulin and TgAb Tests? Causes of Elevated Levels and Their Link to Cancer

What Is Thyroglobulin?

The thyroid is a small gland in the neck. It releases hormones that affect various bodily functions, including helping with metabolism. Thyroglobulin is a glycoprotein produced by follicular cells in the thyroid gland that does not form thyroid hormones and serves a supporting role.

Autoimmune disorders can disrupt thyroglobulin production. Autoimmune disorders occur when the immune system attacks healthy cells with antibodies.

Measuring the protein thyroglobulin (abbreviated Tg) in the blood is an important laboratory test for checking whether a patient still has thyroid tissue. Blood Tg measurements are important because Tg can only be produced by the thyroid gland . When a patient's thyroid has been completely removed, measuring Tg in a blood sample can be used to check whether any tumor remains.

However, many patients still have measurable Tg levels in their blood after surgery. This Tg usually comes from a small amount of normal thyroid tissue left behind. This means that a measurable Tg level does not necessarily indicate the presence of a tumor. Doctors usually give radioactive iodine (atomic) therapy to eliminate the last remaining normal thyroid tissue and make subsequent Tg measurements a better marker for any remaining tumor. 

What Are Thyroglobulin Antibodies (TgAb)?

When the immune system attacks the thyroid, it typically targets thyroglobulin. In doing so, the immune system produces thyroglobulin antibodies (TgAb) . In other words, antibodies formed against Tg are called TgAb.

A TgAb test checks the level of these antibodies in a person's blood. A high level suggests that the person has an autoimmune disorder affecting the thyroid. However, doctors may request other tests to diagnose autoimmune disorders.

Why Is a TgAb Test Performed?

A TgAb test checks for the presence of thyroglobulin antibodies. These antibodies may be a sign of thyroid cancer or an autoimmune disease that damages the thyroid.

Thyroid diseases are primarily managed by endocrinologists , specialists in a subspecialty of internal medicine. Endocrinologists or internal medicine specialists may request a TgAb test to check for Graves' disease or Hashimoto's thyroiditis.

According to a 2019 study, high TgAb levels correspond to more severe symptoms of Hashimoto's thyroiditis. However, a diagnosis is made by evaluating the TgAb result together with other thyroid function tests.

A 2018 study notes that TgAb tests are less effective at predicting thyroid dysfunction than tests that check for thyroid peroxidase antibodies. However, the TgAb test has a broader range of uses, such as the differential diagnosis of thyroid cancers.

How Are Thyroglobulin and TgAb Tests Performed?

A Tg or TgAb test requires a blood sample. The test is performed using immunoassay instruments (automated hormone analyzers).

Before the test

A person may not need to do anything special to prepare for the test. However, they should consult a doctor about taking their medicines, eating, and drinking before the blood test. The doctor may recommend fasting before the test or stopping certain medicines.

During the test

A healthcare professional will clean an area of the arm with an alcohol wipe. After cleaning it, they will insert a needle into a vein and draw a small amount of blood into a tube.

The healthcare professional will likely apply a small amount of pressure to the needle insertion site and check for bleeding. They will then use a dressing to cover the site before sending the sample to a laboratory for analysis.

How Long Do Results Take?

Tg and TgAb test results are usually available on the same day. Patients can access their results through the hospital information system or Turkey's e-Nabız system.

The results of Tg and TgAb tests will show whether a person has high levels of thyroglobulin and thyroglobulin antibodies. Although this may suggest an autoimmune disorder, it may also indicate other conditions, such as thyroid cancer.

A person should review their test results with a doctor. If necessary, the doctor can recommend next steps and additional tests.

What Does an Elevated Thyroglobulin Antibody Test Result Mean?

The "normal" values described below may vary slightly depending on the test kit and instrument used. Guidelines for Endocrinology and Metabolic Diseases are taken into account when making clinical decisions.

In a healthy person:

• The normal value for thyroglobulin (Tg) is 1.5 to 40 ng/mL (nanograms per milliliter). 
• The normal value for thyroglobulin antibodies (TgAb) should be less than 20 IU/mL . When levels are above 20, this usually indicates high levels of TgAb antibodies, meaning that the body's immune system is attacking the thyroid gland.

A high TgAb test result means that a person may have the following problems:

• thyroid damage
• an autoimmune disorder such as Hashimoto's disease
• thyroid cancer

A doctor may recommend additional tests to help diagnose specific conditions.

What Common Thyroid Problems Affect TgAb Test Results?

Some common thyroid problems are listed below. Treating these conditions can help improve a person's quality of life and prevent additional health problems.

Underactive thyroid

Hypothyroidism , also known as an underactive thyroid, means that the gland does not produce enough thyroid hormone to meet the body's needs. Doctors usually prescribe levothyroxine to replace the hormones that the thyroid typically produces.

Overactive thyroid

Hyperthyroidism , also known as an overactive thyroid, produces too much thyroid hormone. This can speed up several different functions in the body. Treatment for hyperthyroidism may include medicines or surgery.

Treatment for hyperthyroidism varies according to the person's age, other health conditions, access to a surgeon, and possible drug allergies.

Chronic thyroiditis

Chronic thyroiditis is also known as Hashimoto's thyroiditis . It is an autoimmune disease that causes inflammation of the thyroid. Treatments may vary, but most people with this condition will need levothyroxine for the rest of their lives.

Subacute thyroiditis

Subacute thyroiditis is an inflammatory condition that affects the thyroid, causing pain and swelling. In some cases, a person may not need treatment. Other treatment options may involve over-the-counter or prescription pain medicines to help with the inflammation.

Thyroid cancer

Do high thyroglobulin antibody levels necessarily mean cancer?

No, because approximately 15 to 20 percent of thyroid cancer patients have circulating antibodies against Tg, known as TgAb.

According to a 2018 study, high thyroglobulin antibody levels may indicate thyroid cancer and may be useful in diagnosis. However, other conditions can also cause elevated levels, so a doctor will need to request additional tests, such as thyroid ultrasound and needle biopsy, to help rule out other causes.

Summary

A TgAb test checks for the presence of thyroglobulin antibodies. For this test, the person will need to provide a small blood sample for laboratory analysis.

If the results show high thyroglobulin antibody levels, this may indicate several conditions affecting the thyroid, including autoimmune conditions and thyroid cancer.

A person who has this test should talk to their doctor about the results. There are many reasons for an elevated TgAb test result, and additional tests may be needed for a definitive diagnosis.