Thyroglobulin (Tg)
A protein made only by thyroid tissue. Its main job is as a tumour marker after treatment for differentiated thyroid cancer: once the thyroid is removed, thyroglobulin should be very low, so a rise can signal returning disease.
What it measures
Thyroglobulin is a large protein produced exclusively by thyroid cells (normal or cancerous) as the scaffold for making thyroid hormone. Because only thyroid tissue makes it, its dominant clinical use is as a tumour marker in people treated for differentiated (papillary or follicular) thyroid cancer: after the thyroid is removed by surgery and often radioactive iodine, there should be little or no thyroid tissue left, so thyroglobulin should fall to very low or undetectable. A measurable or rising level afterwards suggests residual or recurrent cancer and triggers imaging. Crucially, it must always be measured together with anti-thyroglobulin antibodies (anti-Tg), because those antibodies interfere with the test and can make the thyroglobulin read falsely low. It is not used to diagnose thyroid cancer in the first place.
What a high value can mean
- After thyroid-cancer treatment — a measurable or rising thyroglobulin suggests residual or recurrent differentiated thyroid cancer; the key monitoring signal.
- Before treatment / with an intact thyroid — can be raised in many benign conditions (goitre, thyroiditis, Graves', nodules), which is why it is not a diagnostic cancer test.
- Thyroid inflammation or injury — a transient rise.
What a low value can mean
- After total thyroidectomy for cancer — a very low or undetectable thyroglobulin is the desired result, indicating no significant thyroid tissue remains.
- Caution — a low reading is only reliable if anti-Tg antibodies are absent; antibodies can mask disease.
When to discuss with a doctor
Thyroglobulin is a monitoring tool for people already treated for differentiated thyroid cancer, not a screening or diagnostic test for the general population. In that setting, the trend matters most: a persistently undetectable level is reassuring, while a measurable or rising thyroglobulin prompts imaging to look for recurrence and is discussed with the treating team. It must be interpreted with anti-Tg antibodies, since their presence undermines the result. Mediora.AI shows thyroglobulin together with anti-Tg antibodies and thyroid function so the reading is read in context; the cancer-surveillance decisions belong with endocrinology.