Primary hyperparathyroidism causes hypercalcemia (elevated blood calcium levels) through the excessive secretion of parathyroid hormone (PTH), usually by an adenoma (benign tumors) of the parathyroid glands. Its incidence is approximately 1 per 1,000 people (0.1%), while there are 25-30 new cases per 100,000 people per year in the United States. It is almost exactly three times as common in women as men.