Primary hyperparathyroidism is a glandular disorder in which excessive secretion of a hormone called PTH (parathyroid hormone) by one of the parathyroid glands located in the neck causes the calcium levels in the bloodstream to rise. The defect in the parathyroid gland is usually due to a solitary benign tumor in one of the glands. Occasionally, a malignant tumor of one of the parathyroid glands is responsible. Mild elevations of calcium usually cause no clinical symptoms; however, as calcium levels continue to rise, signs of illness develop.

Three body systems are most commonly affected: the gastrointestinal system, the kidneys, and the nervous system. There are many other disorders that can cause elevation in calcium levels. The diagnostic tests recommended will help differentiate hyperparathyroidism from the following:

Cancer. Certain types of cancer can cause an elevation in serum calcium levels.

Rat poison ingestion. There are certain types of rodenticides that work by raising the calcium level in the bloodstream.

Addison's disease. This is a disorder of the adrenal glands that frequently causes high serum calcium.

Chronic kidney failure. Approximately 10 to 20 percent of pets with chronic kidney failure will have elevated calcium levels

Fungal disease. A few fungal disorders have been reported to cause elevations in calcium levels. This is fairly uncommon

Excessive supplementation of the diet with Vitamin D. Vitamin D will increase calcium levels in the blood. Too much vitamin D, given as a dietary supplement, can raise the calcium levels too high