Hyperparathyroidism is often diagnosed before signs or symptoms of the disorder are apparent. When symptoms do occur, they're the result of damage or dysfunction in other organs or tissues due to high calcium levels circulating in the blood and urine or too little calcium in bones.

Symptoms may be so mild and nonspecific that they don't seem at all related to parathyroid function, or they may be severe. The range of signs and symptoms include:

• Fragile bones that easily fracture (osteoporosis)
• Kidney stones
• Excessive urination
• Abdominal pain
• Tiring easily or weakness
• Depression or forgetfulness
• Bone and joint pain
• Frequent complaints of illness with no apparent cause
• Nausea, vomiting or loss of appetite

Hyperparathyroidism is caused by factors that increase the production of parathyroid hormone.

The parathyroid glands maintain proper levels of both calcium and phosphorus in your body by turning the secretion of parathyroid hormone (PTH) off or on, much like a thermostat controls a heating system to maintain a constant air temperature. Vitamin D also is involved in regulating the amount of calcium in your blood.

Normally, this balancing act works well. When calcium levels in your blood fall too low, your parathyroid glands secrete enough PTH to restore the balance. PTH raises calcium levels by releasing calcium from your bones and increasing the amount of calcium absorbed from your small intestine.

When blood-calcium levels are too high, the parathyroid glands produce less PTH. But sometimes one or more of these glands produce too much hormone, leading to abnormally high levels of calcium (hypercalcemia) and low levels of phosphorus in your blood.

The mineral calcium is best known for its role in keeping your teeth and bones healthy. But calcium has other functions. It aids in the transmission of signals in nerve cells, and it's involved in muscle contraction. Phosphorus, another mineral, works in conjunction with calcium in these areas.

The disorder can generally be divided into two types based on the cause. Hyperparathyroidism may occur because of a problem with the parathyroid glands themselves (primary hyperparathyroidism) or because of another disease that affects the glands' function (secondary hyperparathyroidism).

Primary hyperparathyroidism

Primary hyperparathyroidism occurs because of some problem with one or more of the four parathyroid glands:

• A noncancerous growth (adenoma) on a gland is the most common cause.
• Enlargement (hyperplasia) of two or more parathyroid glands accounts for most other cases.
• A cancerous (malignant) tumor is a rare cause of primary hyperparathyroidism.

Primary hyperparathyroidism usually occurs randomly, but some people inherit a gene that causes the disorder.

Secondary hyperparathyroidism

Secondary hyperparathyroidism is the result of another condition that lowers calcium levels. Therefore, your parathyroid glands overwork to compensate for the loss of calcium. Factors that may contribute to secondary hyperparathyroidism include:

• Severe calcium deficiency. Your body may not get enough calcium from your diet, often because your digestive system doesn't absorb the calcium from it.
• Severe vitamin D deficiency. Vitamin D helps maintain appropriate levels of calcium in the blood, and it helps your digestive system absorb calcium from your food.

Your body produces vitamin D when your skin is exposed to sunlight, and you consume some vitamin D in food. If you don't get enough vitamin D, then calcium levels may drop.

• Chronic kidney failure. Your kidneys convert vitamin D into a form that your body can use. If your kidneys function poorly, usable vitamin D may decline and calcium levels drop. Chronic kidney failure is the most common cause of secondary hyperparathyroidism.

You may be at an increased risk of primary hyperparathyroidism if you:

• Are a woman who has gone through menopause
• Have had prolonged, severe calcium or vitamin D deficiency
• Have a rare, inherited disorder, such as multiple endocrine neoplasia, type 1, which usually affects multiple glands
• Have had radiation treatment for cancer that has exposed your neck to radiation
• Have taken lithium, a drug most often used to treat bipolar disorder

Complications of hyperparathyroidism are primarily related to the long-term effect of too little calcium in your bones and too much calcium circulating in your bloodstream. Common complications include:

• Osteoporosis. The loss of calcium often results in weak, brittle bones that fracture easily (osteoporosis).
• Kidney stones. The excess of calcium in your blood may lead to excess calcium in your urine, which can cause small, hard deposits of calcium and other substances to form in your kidneys. A kidney stone usually causes significant pain as it passes through the urinary tract.
• Cardiovascular disease. Although the exact cause-and-effect link is unclear, high calcium levels are associated with cardiovascular conditions, such as high blood pressure (hypertension) and certain types of heart disease.
• Neonatal hypoparathyroidism. Severe, untreated hyperparathyroidism in pregnant women may cause dangerously low levels of calcium in newborns.