Causes of hypercalcemia include:
- Overactivity of parathyroid glands. The primary cause of hypercalcemia is overactivity in one or more of your four parathyroid glands (primary hyperparathyroidism), which lie behind your thyroid gland in your neck.
- Cancer. Certain types of cancer, particularly lung cancer and breast cancer, as well as some cancers of the blood, such as multiple myeloma, increase your risk of hypercalcemia. Some cancerous (malignant) tumors produce a protein that acts like parathyroid hormone, stimulating the release of calcium from your bones into your blood. This is considered a paraneoplastic syndrome, your body's response to the presence of cancer or a substance the cancer produces. Spread of cancer (metastasis) to your bones also increases your risk of hypercalcemia.
- Other diseases. Some diseases that produce areas of inflammation due to tissue injury (granulomas) may raise blood levels of vitamin D (calcitriol). Granulomatous diseases include tuberculosis, an infectious lung disease, and sarcoidosis, an inflammatory disease that usually begins in your lungs. Elevated levels of calcitriol stimulate your digestive tract to absorb more calcium, which raises the level of calcium in your blood. Also, 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.
- Disease effects. People with cancer or other diseases that cause them to spend a great deal of time sitting or lying down may develop hypercalcemia. Over time, bones that don't bear weight release calcium into the blood.
- Medications. Certain drugs such as lithium, which is used to treat bipolar disorder may increase the release of parathyroid hormone and cause hypercalcemia. Thiazide diuretics can cause elevated calcium levels in your blood by decreasing the amount of calcium you excrete in your urine.
- Supplements. Eating or drinking too much calcium or vitamin D supplements over time can raise calcium levels in your blood above normal.
- Dehydration. A common cause of mild or transient hypercalcemia is dehydration, because when there is less fluid in your blood, calcium concentrations rise.
How calcium affects your body
Your body stores calcium mainly in your bones, but also in certain cells, particularly in your muscles and your blood. When you eat calcium-rich foods such as milk, cheese and leafy green vegetables, your body usually rids itself of any excess when you urinate, which maintains a normal level of calcium in your blood.
Two hormones serve as primary regulators of the calcium in your blood: parathyroid hormone and calcitonin.
When the calcium in your blood falls, your body produces more parathyroid hormone; when your calcium blood level rises, your body produces less of the hormone. In a finely tuned system of checks and balances, parathyroid hormone causes:
- 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
Normally, if the calcium level in your blood rises too high, your thyroid gland produces calcitonin, a hormone that slows the release of calcium from your bones. The balance is thrown off in hypercalcemia, and your body can't counter the effects of too much calcium as it usually does.