The type I form of multiple endocrine neoplasia typically affects the:
- parathyroid gland
- pancreatic islets
- anterior pituitary gland
The gene for MEN-I has been mapped to chromosome 11q13.
Parathyroid tumours occur in over 90% of cases resulting in hypercalcaemia and sometimes, clinical hyperparathyroidism.
Multiple tumours of the endocrine pancreas are present in up to 75% of cases. Gastrinoma leading to the Zollinger - Ellison syndrome is the largest single cause of morbidity and mortality in MEN - I. Other tumours include insulinoma, and rarely, vipoma and glucagonoma.
Anterior pituitary gland tumours are present in about 50% of cases. Most secrete either prolactin, growth hormone or adrenocorticotrophin with associated symptoms of amenorrhoea or galactorrhoea, acromegaly or Cushing's syndrome respectively. Others are clinically non-functional and produce symptoms of pituitary hypofunction or visual field loss.
Occasionally, carcinoid tumours and tumours of the adrenal cortex may occur.
Often, only one major lesion is evident clinically but other asymptomatic tumours are present and discovered when investigated directly. The high occurrence of parathyroid tumours and hypercalcaemia make serum calcium a useful indicator of MEN-I in suspected individuals.