On the other hand, functioning tumors secrete hormones related to a specific clinical syndrome, such as insulin (insulinoma), glucagon (glucagonoma, confirmed by serum glucagon level > 1000 pg/mL), gastrin (elevated serum gastrin and gastric acids), and vasoactive intestinal polypeptide (vipoma, VIP values > 200 pg/mL) [19–21]. Here, GAST is linked to pancreatic insulinoma.