Parathyroid disorders may result from a number of factors. The causes of hyperparathyroidism tend to be very different from those of hypoparathyroidism.

Possible causes of hyperparathyroidism include:

• Adenomas. Benign (noncancerous) tumors. An adenoma on a parathyroid gland can cause it to become overactive. Most cases of hyperparathyroidism result from a single adenoma, but some patients have more than one. Adenomas cause 85 percent of cases of primary hyperparathyroidism, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
  
  
• Parathyroid hyperplasia. Enlargement of the parathyroid glands. This enlargement makes the glands release additional parathyroid hormone (PTH).
  
  
• Parathyroid cyst or cancer. Both are rare, but can cause the excretion of excess PTH.
  
  
• Genetic defects. In some cases, hyperparathyroidism may occur in families, indicating a possible genetic link. Familial hypocalciuric hypercalcemia is a form of hyperparathyroidism that is linked to an identified genetic mutation. As a result of the mutation, the parathyroid glands are unable to detect calcium in the blood and continually release PTH.
  
  
• Radiation. Exposure to radiation, especially when the patient is young, is thought to contribute to hyperparathyroidism.
  
  
• Multiple endocrine neoplasia type 1 (MEN1). A rare, inherited group of conditions. Growths occur on the parathyroid glands, making all four overactive. In addition to hyperparathyroidism, MEN1 may also cause other endocrine disorders, such as insulinoma, an insulin-producing tumor of the pancreas.
  
  

Certain medications may also cause or worsen hyperparathyroidism. Lithium, used primarily to treat bipolar disorder (manic depression), can alter the body’s use of calcium and its response to PTH, leading to hyperparathyroidism. In addition, thiazide diuretics, used to treat high blood pressure and other conditions, may also aggravate hyperparathyroidism.

Hyperparathyroidism may also be the result of other conditions, in which case it is known as secondary hyperparathyroidism. Causes of secondary hyperparathyroidism include chronic kidney failure, absorption disorders of the intestines and kidneys, and rickets (a bone disease related to insufficient vitamin D). Patients on long-term hemodialysis, the most common treatment for chronic kidney failure, may be at additional risk.

Women are twice as likely than men to develop hyperparathyroidism, according to the National Institutes of Health. Additionally, the risk of developing hyperthyroidism increases with age, with most cases first identified in patients older than 45 years.

Hypoparathyroidism is rare and most often results from damage to parathyroid glands after surgery or traumatic injury to the neck area. For example, hungry bone syndrome is a condition in which the bones conserve calcium in an effort to heal after removal of the parathyroid glands (parathyroidectomy), resulting in hypoparathyroidism. Certain treatment methods for hyperparathyroidism, such as radioactive iodine treatment, can also cause hypoparathyroidism.

Other causes of hypoparathyroidism can include:

• Autoimmune diseases. In some cases, the body creates antibodies that attack the parathyroid glands. This may damage the glands and render them dysfunctional.
  
  
• DiGeorge syndrome. A congenital condition in which the patient is born without parathyroid glands. DiGeorge syndrome also causes heart and immune system problems.
  
  
• Low levels of magnesium in the blood. This can interfere with the ability to produce PTH.
  
  
• Type 1 autoimmune polyglandular syndrome. A cause of familial hypoparathyroidism. This is a disease in which the immune system attacks the endocrine system. Patients may also have Addison’s disease, other autoimmune diseases and chronic yeast infections.