When an abnormal parathyroid gland ‘goes rogue’ and stops responding to normal signals from the body, it tells the body (unnecessarily) that it needs more calcium. Calcium is then leached from the bones (a cause of osteoporosis) and intestines, and can be deposited in the kidneys (leading to kidney stones) or other areas of the body. A build-up of too much calcium in the bloodstream can be either asymptomatic or can lead to a variety of symptoms.
Some patients with a parathyroid tumor have normocalcemic hyperparathyroidism where the calcium level is normal but the PTH is high. Other patients may have high calcium but high-normal or normal PTH levels. Assuming that there are no other causes of high calcium or high PTH, these patients ALL have primary hyperparathyroidism that should generally be treated.
The cause of overactive parathyroids in 1% of patients is inheritance of a gene abnormality from one parent. The cause of a parathyroid tumor in the other 99% of patients is usually unknown, although radiation therapy to the face or neck can be a factor in causing a single parathyroid tumor.
Secondary hyperparathyroidism is when a disease unrelated to your parathyroids causes all of your parathyroid glands to become enlarged and overactive. The most common cause is kidney failure, which at least in the beginning, leads to lower levels of calcium in the bloodstream that stimulates the parathyroid glands.
Prolonged vitamin D deficiency is the second most common of secondary hyperparathyroidism, followed by gastrointestinal malabsorption (such as after a gastric bypass procedure).
Tertiary hyperparathyroidism is when a patient that previously had secondary hyperparathyroidism now develops primary hyperparathyroidism, ie. A kidney failure patient gets a kidney transplant and some years later is diagnosed with primary hyperparathyroidism (when their new kidney is still working).
If it is true tertiary hyperparathyroidism, then the statistics are the same as primary hyperparathyroidism, ie most people have a single overactive parathyroid gland. If however, the secondary hyperparathyroidism was never ‘cured’ and then patient still has some renal insufficiency, then these patients likely have disease in all four of their parathyroid glands.
Parathyroid cancer is a rare cause of primary hyperparathyroidism. It is very difficult to make a parathyroid cancer diagnosis before surgery. Typically (but not always), patients will have very high calcium and very high PTH levels.
During parathyroid surgery, the parathyroid tumor is usually hard and stuck to the surrounding tissues and thyroid gland. Parathyroid cancer survival is generally very good since the tumor is very slow-growing, but since it can come back, patients may need re-operations in the future.