Thyroglobulin (TG) Test

The thyroglobulin (TG) test is used to measure the amount of thyroglobulin in a sample of blood. The test is also known as the TG test, TGB test, and thyroglobulin tumour marker test.

Thyroglobulin is a key protein in the thyroid gland needed to produce thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3). Thyroid hormones work together to control your heart rate and metabolism (the breakdown of food). Usually, the thyroid gland releases small amounts of thyroglobulin into your bloodstream.

However, in patients with differentiated thyroid cancer (such as papillary carcinoma and follicular thyroid cancer), the cancer cells can produce excess thyroglobulin. Therefore, doctors use thyroglobulin as a tumour marker to:

• Monitor treatment response.
• Detect recurrence after surgery or therapy.
• Assess the effectiveness of ongoing management in thyroid cancer patients.

You may be advised to get the thyroglobulin test if you experience the following symptoms of:

• Lump in the neck (neck swelling or nodule)
• Difficulty breathing
• Difficulty or pain while swallowing
• Changes in your voice (hoarseness)
• Persistent cough or swollen lymph nodes

The thyroglobulin test is highly recommended for the following categories of people:

• Individuals who have been diagnosed with differentiated thyroid cancer (especially papillary or follicular types): The TG test helps monitor disease progression, response to treatment, and risk of recurrence.
• Individuals who have undergone thyroid cancer treatment (surgery, radioiodine therapy, etc.): After successful treatment, thyroglobulin levels should be low or undetectable. The test is crucial for detecting residual disease or cancer recurrence.

Although the thyroglobulin test alone cannot diagnose thyroid cancer, it is primarily used to monitor patients with known differentiated thyroid cancer:

• Papillary carcinoma – This is the most common type of thyroid cancer, which begins with a small lump in the thyroid gland. While the cause is unknown, genetics might increase the risk of getting this disease.
• Follicular thyroid cancer – This refers to a tumour in certain cells of the thyroid gland, known as follicular cells. It is the second most common type of thyroid cancer.

The test may also be used with other tests to help diagnose and monitor conditions such as:

• Hyperthyroidism – In this condition, the thyroid gland is overactive and produces excess thyroid hormones, including thyroglobulin levels may also be elevated.
• Hypothyroidism – In this condition, the thyroid gland is underactive, leading to insufficient thyroglobulin production.
• Thyroiditis – This is a condition characterised by inflammation of the thyroid gland, which can lead to a temporary rise in thyroglobulin levels.

What to Expect Before the Thyroglobulin (TG) Test

No fasting is required for a standalone Thyroglobulin (TG) test. You can eat and drink normally. However, if it is part of a broader package that includes fasting tests, your doctor may advise fasting for 8-12 hours. Avoid strenuous exercise for 24 hours before the test, as intense physical activity can temporarily raise Thyroglobulin (TG) levels and affect interpretation. Always inform your doctor about any ongoing medications.

What to Expect During the Blood Collection

A phlebotomist cleans the skin with an antiseptic, then places an elastic band around your upper arm to make the veins easier to access. A small needle is inserted into the vein; you may feel a brief pinch. Blood is collected into a labelled tube in under a minute.

What to Expect After the Thyroglobulin (TG) Test

A small cotton swab and bandage are placed over the site to stop any minor bleeding. You can return to normal activities immediately. Minor bruising is normal and disappears quickly. If dizzy, sit and rest for a few minutes.

In the thyroglobulin test, the primary parameter being measured is:

• The level of thyroglobulin in a sample of blood.

The normal thyroglobulin levels are indicated in the table below:

Note: The normal range may vary slightly depending on the laboratory, assay method, and individual health conditions (e.g., the presence of thyroglobulin antibodies). After total thyroidectomy, levels are expected to be low or undetectable.

The table below provides a general interpretation of the TG lab test results based on whether the levels are low or high over time.

Note:

• Your doctor will likely check your thyroglobulin levels over time instead of making conclusions based on a single test result. This is because other thyroid conditions and certain illnesses can affect your levels.
• The test results should always be interpreted along with the patient’s medical history, clinical examination, and other findings.

Like any other medical test, this test also has certain risks and limitations.

Risks:

The thyroglobulin test is generally safe and involves minimal risk. You may experience slight pain when the needle is inserted or mild bruising afterwards, which usually resolves spontaneously.

Limitations:

• A thyroglobulin test may not be reliable if you have anti-thyroglobulin antibodies in your blood. These are antibodies (proteins produced in response to foreign invaders) that can interfere with the test and cause its levels to appear falsely low.
• Understanding thyroglobulin test results can be complicated. Test results depend on your medical history, the type of treatment you receive, and the results of repeat tests done at the same lab.

Depending on your symptoms and results, your doctor might recommend certain additional tests to better understand your condition:

• Thyroid antibody test
• Calcitonin test
• Carcinoembryonic antigen (CEA) test
• TSH (thyroid-stimulating hormone) test
• Tumour marker tests
• Thyroid ultrasound
• PTEN genetic test