Thyroglobulin (Tg) is a protein produced exclusively by the thyroid gland, a small, butterfly-shaped organ at the base of the neck. Measuring its amount in the blood provides important information about thyroid health, primarily to monitor specific thyroid conditions.
Thyroglobulin: An Overview
Thyroglobulin is a large glycoprotein synthesized within thyroid cells. Its main function involves the synthesis and storage of thyroid hormones (triiodothyronine (T3) and thyroxine (T4)). These hormones are crucial for regulating the body’s metabolism, affecting heart rate, temperature, and how the body uses energy.
The thyroid gland stores thyroglobulin in its follicular lumens to form thyroid hormones. Only a small amount is typically released into the bloodstream in healthy individuals. Normal levels generally range from 3 to 40 nanograms per milliliter (ng/mL), though this can vary slightly between laboratories.
Why Thyroglobulin Levels Are Measured
The primary reason for measuring thyroglobulin levels is to monitor patients diagnosed with differentiated thyroid cancer, specifically papillary and follicular types. Following treatments like total thyroidectomy, which involves removing the entire thyroid gland, and often radioactive iodine therapy, thyroglobulin serves as a tumor marker. Its presence in the blood after gland removal can indicate the persistence or recurrence of thyroid cancer cells.
Regular testing helps evaluate the effectiveness of the initial cancer treatment and allows for early detection of any remaining or returning cancer cells. This monitoring is a long-term process, with tests often performed periodically over several years.
Thyroglobulin levels may also be measured to assess certain non-cancerous thyroid conditions, such as inflammatory conditions like thyroiditis, or an enlarged thyroid gland (goiter). However, for these conditions, thyroglobulin is not a primary diagnostic tool, and its main utility remains in the surveillance of differentiated thyroid cancer.
Understanding Thyroglobulin Test Results
Interpreting thyroglobulin test results depends significantly on the clinical context, especially whether the thyroid gland has been surgically removed. For individuals who have undergone total thyroidectomy for differentiated thyroid cancer, undetectable or very low thyroglobulin levels are generally a favorable sign. This suggests that the treatment successfully removed all thyroid tissue, including any cancerous cells.
Conversely, detectable or rising thyroglobulin levels after thyroidectomy are a concern, as they can signal the presence of residual thyroid tissue or the recurrence or spread of thyroid cancer. Even low but increasing levels can indicate disease progression, prompting further evaluation with imaging studies like neck ultrasound. The American Thyroid Association guidelines suggest that a stimulated thyroglobulin level below 1.0 ng/mL is associated with a high probability of being tumor-free.
In individuals with an intact thyroid gland, elevated thyroglobulin levels can result from various benign conditions. These may include inflammation of the thyroid (thyroiditis), overactive thyroid (Graves’ disease), or large goiters. Because thyroglobulin is released whenever thyroid tissue is stimulated or damaged, its levels can be high. Therefore, in patients with an intact thyroid, the test is not typically used to diagnose cancer but rather to assess overall thyroid activity or injury.
Related Thyroglobulin Testing
When measuring thyroglobulin, it is crucial to also test for thyroglobulin antibodies (TgAb). These are proteins produced by the body’s immune system that can mistakenly target thyroglobulin. The presence of TgAb is common in autoimmune thyroid diseases like Hashimoto’s thyroiditis and Graves’ disease, but they can also be found in individuals with thyroid cancer.
The primary concern with TgAb is their ability to interfere with the accurate measurement of thyroglobulin levels. These antibodies can bind to thyroglobulin, leading to falsely low or even undetectable thyroglobulin results, which might mask the presence of residual or recurrent cancer. For this reason, laboratories typically measure TgAb alongside thyroglobulin to ensure proper interpretation of the results. If TgAb are present, their own levels can sometimes serve as a marker for disease activity, especially in the context of thyroid cancer surveillance.