Causes

Besides building strong bones and teeth, calcium helps muscles contract and nerves transmit signals. Normally, if there isn't enough calcium in your blood, your parathyroid glands secrete a hormone that triggers:

• Your bones to release calcium into your blood
• Your digestive tract to absorb more calcium
• Your kidneys to excrete less calcium and activate more vitamin D, which plays a vital role in calcium absorption

This delicate balance between too little calcium in your blood and hypercalcemia can be disrupted by a variety of factors. Hypercalcemia is caused by:

• Overactive parathyroid glands (hyperparathyroidism). This most common cause of hypercalcemia can stem from a small, noncancerous (benign) tumor or enlargement of one or more of the four parathyroid glands.
• Cancer. Lung cancer and breast cancer, as well as some blood cancers, can increase your risk of hypercalcemia. Spread of cancer (metastasis) to your bones also increases your risk.
• Other diseases. Certain diseases, such as tuberculosis and sarcoidosis, can raise blood levels of vitamin D, which stimulates your digestive tract to absorb more calcium.
• Hereditary factors. A rare genetic disorder known as familial hypocalciuric hypercalcemia causes an increase of calcium in your blood because of faulty calcium receptors in your body. This condition doesn't cause symptoms or complications of hypercalcemia.
• Immobility. People who have a condition that causes them to spend a lot of time sitting or lying down can develop hypercalcemia. Over time, bones that don't bear weight release calcium into the blood.
• Severe dehydration. A common cause of mild or transient hypercalcemia is dehydration. Having less fluid in your blood causes a rise in calcium concentrations.
• Medications. Certain drugs — such as lithium, used to treat bipolar disorder — might increase the release of parathyroid hormone.
• Supplements. Taking excessive amounts of calcium or vitamin D supplements over time can raise calcium levels in your blood above normal.