Serum thyroglobulin (serum-Tg)
As mentioned above, thyroglobulin (Tg), the hormone storage form in the thyroid gland, is also present in blood in small amounts. Tg is produced by both healthy and degenerate thyroid gland cells. A drop in serum Tg to 0 after an operation and radiotherapy of a follicular or papillary thyroid gland carcinoma confirms the complete absence of thyroid-gland cells in the patient’s body, and thus the success of the therapy. Any future reappearance of serum Tg is a sign of renewed tumor activity. Repeated measurements of serum Tg at prescribed time intervals allows very early detection of a tumor recurrence. In this context, serum Tg is termed a tumor marker.
Human calcitonin (hCt)
Human calcitonin (hCt) is a protein hormone that promotes the uptake of calcium from the blood into the bones, causing a corresponding reduction in the blood calcium level. It plays an important role mainly during growth. The hormone hCt is produced by the C-cells in the thyroid gland, which are not involved in T3 and T4 production and are not under TSH control. With medullary thyroid cancer (C-cell carcinoma), serum hCt level is high. After successful removal of a medullary thyroid cancer, there should be no more hCt in the blood, or levels should lie within the normal range. In this context, high hCt levels are suspicious and are considered tumor markers.
Carcinoembryonic antigen (CEA)
CEA is a protein produced in fetal tissue. After birth it is only produced in small amounts by cells in the intestinal mucosa, liver, and pancreas. In addition to hCt, most C-cell tumors also produce CEA. Unlike hCt, CEA is not a hormone, however, it is also used as a tumor marker for c-cell cancers and is measured during aftercare.