In about 80 percent of people with primary hyperparathyroidism, a benign, or noncancerous, tumor called an adenoma has formed in one of the parathyroid glands. The tumor causes the gland to become overactive. In most other cases, the excess hormone comes from two or more overactive parathyroid glands, a condition called multiple tumors or hyperplasia. Rarely, primary hyperparathyroidism is caused by cancer of a parathyroid gland.

In most cases, health care providers don’t know why adenoma or multiple tumors occur in the parathyroid glands. Most people with primary hyperparathyroidism have no family history of the disorder, but some cases can be linked to an inherited problem. For example, familial multiple endocrine neoplasia type 1 is a rare, inherited syndrome that causes multiple tumors in the parathyroid glands as well as in the pancreas and the pituitary gland. Another rare genetic disorder, familial hypocalciuric hypercalcemia, causes a kind of hyperparathyroidism that is atypical, in part because it does not respond to standard parathyroid surgery.