Primary hyperparathyroidism is a parathyroid disorder. It is characterized by an excess production of the parathyroid hormone. When this occurs, calcium blood levels become elevated and levels of vitamin D are typically lower. It is important to distinguish the difference between vitamin D deficiency and primary hyperparathyroidism. The parathyroid glands are part of the endocrine system. Located in the neck, there are typically four tiny parathyroid glands about the size of a grain of rice. If affected by a disorder, they can abnormally grow to the size of a grape. Along with vitamin D, parathyroid hormone is the main regulator of the level of calcium in the blood. It also affects the level of phosphorus in the blood, bone cell activity and bone growth. Most individuals with primary hyperparathyroidism are asymptomatic (do not develop symptoms) or only exhibit extremely mild symptoms. While primary hyperparathyroidism mainly affects the kidneys and the skeleton, other areas of the body such as the gastrointestinal tract, heart, and nervous system can sometimes be involved. In the majority of cases (approximately 80 to 85 percent), the disorder is caused by a benign tumor called an adenoma affecting one of the four parathyroid glands.