Hyperfunction of the parathyroid glands can be categorised according to the aetiology in primary and secondary forms. In primary hyperparathyroidism (pHPT), the cause of overactivity of the parathyroid glands lies with the organ itself, while secondary hyperparathyroidism (sHPT) is a disorder of the calcium phosphate metabolism caused by renal or gastrointestinal disorders. People with familial pHPT are a small but important group (about 5% of all cases with pHPT). These familial syndromes include the multiple endocrine neoplasias (types I, IIA and IV), the familial hypocalciuric hypercalcaemia (FHH) and the neonatal severe pHPT. About 2% of all cases with pHPT are caused by MEN 1. This syndrome is defined by tumours of the parathyroid glands, the islet cells of the pancreas and the hypophysis. The tumours are removed surgically.