tokens: Specific neuroendocrine biomarkers that are used in the diagnosis of functional NENs include insulin , glucagon , vasoactive intestinal polypeptide , gastrin , somatostatin , adrenocorticotropin , growth hormone releasing hormone , parathyroid hormone - related peptide , serotonin , histamine , and 5 - hydroxy indole acetic acid ( 5 - HIAA ) .