Familial isolated hyperparathyroidism (FIHP) is an inherited form of primary hyperparathyroidism that is not associated with other features. In FIHP, tumors involving the parathyroid glands cause the production and release of excess parathyroid hormone, which in turn causes increased calcium in the blood (hypercalcemia). The tumors are usually benign, but a cancerous tumor can develop in rare cases. Abnormal levels of calcium cause many of the symptoms of FIHP, including kidney stones, nausea, vomiting, high blood pressure (hypertension), weakness, and fatigue. Osteoporosis often also develops. FIHP may be caused by genetic changes in the MEN1, CDC73 (also known as the HRPT2 gene), or CASR genes and is typically inherited in an autosomal dominant manner. In some cases, the cause is unknown. Genetic changes in the MEN1 and CDC73 genes cause other conditions in which hyperparathyroidism is one of many features, but some people with genetic changes in these genes have only isolated hyperparathyroidism.