Multiple endocrine neoplasia type 1 (MEN1) includes varying combinations of more than 20 endocrine and non-endocrine tumors. Endocrine tumors become evident either by overproduction of hormones by the tumor or by growth of the tumor itself. Parathyroid tumors are the most common MEN1-associated endocrinopathy; onset in 90% of individuals is between ages 20 and 25 years with hypercalcemia evident by age 50 years; hypercalcemia causes lethargy, depression, confusion, anorexia, constipation, nausea, vomiting, diuresis, dehydration, hypercalciuria, kidney stones, increased bone resorption/fracture risk, hypertension, and shortened QT interval. Pituitary tumors include prolactinoma (the most common), which manifests as oligomenorrhea/amenorrhea and galactorrhea in females and sexual dysfunction in males. Well-differentiated endocrine tumors of the gastro-entero-pancreatic (GEP) tract can manifest as Zollinger-Ellison syndrome (gastrinoma); hypoglycemia (insulinoma); hyperglycemia, anorexia, glossitis, anemia, diarrhea, venous thrombosis, and skin rash (glucagonoma); and watery diarrhea, hypokalemia, and achlorhydria syndrome (vasoactive intestinal peptide [VIP]-secreting tumor). Carcinoid tumors are non-hormone-secreting and can manifest as a large mass after age 50 years. Adrenocortical tumors can be associated with primary hypercortisolism or hyperaldosteronism. Non-endocrine tumors include facial angiofibromas, collagenomas, lipomas, meningiomas, ependymomas, and leiomyomas.

Insulinomatosis and diabetes mellitus syndrome is an autosomal dominant disorder in which affected individuals within a family present with either hyperinsulinemic hypoglycemia secondary to pancreatic neuroendocrine tumors, or a noninsulin-dependent form of diabetes mellitus. A few affected individuals show only impaired glucose tolerance. Some patients also exhibit congenital cataract and/or congenital glaucoma (Iacovazzo et al., 2018).