Thyroglobulin (Tg) is a protein made by the follicular cells within your thyroid gland, a butterfly-shaped organ at the base of your neck. It acts as a foundational structure for the creation and storage of thyroid hormones, thyroxine (T4) and triiodothyronine (T3).
Hormone production involves iodine binding to Tg. Iodinated Tg is stored within thyroid follicles. When the body requires thyroid hormones, Tg is broken down to release T3 and T4 into the bloodstream, regulating bodily functions like heart rate and calorie burning.
Thyroglobulin as a Cancer Marker
Thyroglobulin (Tg) monitors certain thyroid cancers, primarily after surgical removal of the thyroid gland. Both healthy and most differentiated thyroid cancer cells (e.g., papillary, follicular) produce Tg. Measuring blood Tg levels is a valuable surveillance tool, not for initial cancer diagnosis.
After a total thyroidectomy, minimal to no Tg should be present in the bloodstream. If Tg levels are detected or rise after surgery, it can indicate remaining thyroid tissue, benign remnants, or recurrent cancer cells. Therefore, Tg blood tests are a regular part of post-treatment follow-up.
Interpreting Thyroglobulin Levels in Cancer Monitoring
Interpreting Tg levels after thyroid cancer treatment requires considering the patient’s history and result trends. No single number definitively indicates cancer; patterns and thresholds guide clinical decisions.
For patients with a removed thyroid, an “undetectable” or very low Tg level is a favorable sign, suggesting successful elimination of thyroid tissue, including cancer cells. Guidelines consider a stimulated Tg below 1.0 nanograms per milliliter (ng/mL) or suppressed Tg below 0.2 ng/mL as an excellent response.
Conversely, if Tg levels remain persistently detectable, rise, or become elevated after treatment, it suggests that some thyroid tissue, potentially cancerous, is still present or has recurred. Even if initial levels were low, a subsequent increase can signal cancer recurrence. Changes in levels over time provide the most meaningful information.
Factors Influencing Thyroglobulin Results
Several factors influence Tg test results, making interpretation complex.
Residual normal thyroid tissue, even small amounts left after surgery, can produce Tg, leading to detectable levels not necessarily indicative of cancer recurrence.
Anti-thyroglobulin antibodies (TgAb) can interfere with Tg measurement, often leading to falsely low or undetectable readings. TgAb levels are always measured alongside Tg, as their presence necessitates caution in interpreting the Tg result.
Thyroid-stimulating hormone (TSH) also plays a role, as TSH stimulates Tg production. Higher TSH levels increase Tg from any thyroid cells. Therefore, Tg measurements are often performed when TSH is intentionally elevated (stimulated Tg) to maximize detection.
Next Steps After Thyroglobulin Testing
Concerning Tg levels warrant further investigation. Follow-up actions depend on the individual’s clinical context, previous test results, and other health factors.
Imaging, particularly neck ultrasound, is a common next step to identify suspicious nodules or enlarged lymph nodes. CT or PET scans may be used for suspected widespread disease or inconclusive ultrasound findings. If a suspicious area is identified, a biopsy might be performed to determine if cancer cells are present. These diagnostic steps, guided by Tg results, help formulate appropriate management.