Although calcitonin is the most reliable tumor marker due to its high sensitivity and specificity, there are some other proteins that may be released by the malignant transformed C cell.
Serum CEA is usually elevated when the disease is diffuse and distant metastases are present [68,69]. Unlike calcitonin, CEA does not show any response to the pentagastrin stimulus. It is most useful in monitoring the progression of the disease since its level increases when the disease becomes rapidly progressive.
Serum chromogranin may also be elevated in patients with medullary thyroid carcinoma. It is not specific since elevated values have been reported in patients with neither clinical nor biochemical evidence of a primary medullary thyroid carcinoma .
As in many other neuroendocrine tumors, somatostatin, gastrin-releasing peptide, vasoac-tive intestinal peptide, neuron-specific enolase, and other neuroendocrine substances may be produced abnormally but none of these pep-tides are useful for diagnosis [105-107].
Some of the products of medullary thyroid carcinoma may result in significant clinical manifestations: not just CGRP, but also vasoac-tive intestinal peptide, serotonin and prostag-
landins, may all contribute to the flushing and diarrhea syndrome [108,109].
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