Elevated plasma calcitonin gene-related peptide and the symptoms associated with medullary thyroid cancer.

Calcitonin gene-related peptide (CGRP) is a potent vaso-active peptide. Typically, it is elevated in the plasma of patients with medullary thyroid cancer (MTC), but is undetectable in the plasma of healthy individuals. Some symptoms associated with MTC could be caused by elevated CGRP. The plasma level of CGRP was measured in patients with recurrent MTC at rest and again after intravenous pentagastrin to further elevate the CGRP. When normal subjects received pentagastrin the CGRP level still remained below the detectable threshold (< 4 pmol/L). The CGRP levels did not correlate with the patients' blood pressure but a rise in CGRP was associated with a rise in heart rate. As not all patients showed symptoms, further investigation examined the possibility of plasma elevation of somatostatin, a known inhibitor, preventing responses but no change was found. Similarly, examination of the patient plasma for CGRP fractions did not reveal any increased breakdown.