High levels of calcium in the blood, or hypercalcemia, is ultimately due to a hormone imbalance. The primary cause of the imbalance is overproduction of parathyroid hormone (PTH), which speeds up the release of calcium from your bones into your bloodstream. Normally this process would be kept in check by the release of another hormone, calcitonin, which slows down the release of calcium from bones. When you have high blood calcium, calcitonin is not keeping up with PTH. Several conditions can cause this imbalance, which, if left uncorrected, can lead to abnormal heart rhythms, osteoporosis, kidney stones and kidney failure.
Overactive Glands
An overactive parathyroid gland is the most common cause of high blood calcium. One or more parathyroid glands become enlarged and produce excess PTH. When at least two glands are involved, this condition is called hyperplasia. High blood calcium due to excess PTH can also result from a benign (noncancerous) growth on a parathyroid gland.
Supplement Overload
Hypercalcemia from calcium consumption only occurs from excessive use of supplements. (It is technically possible, but not likely, to develop hypercalcemia from drinking too much milk or eating too many dairy products that are naturally high in calcium.) There is not enough calcium in a multivitamin and mineral supplements to cause hypercalcemia, but there is a risk of overdosing from individual calcium supplements. The recommended amount of calcium per day is 1,000 mg for adult men and women and 1,200 mg for those over 50.
Overuse of vitamin D supplements can also result in hypercalcemia because vitamin D plays an important role in maintaining a proper balance of calcium in bones and blood. Recognizing hypercalcemia's potential for serious health problems, the U.S. government's Food and Nutrition Board, which determines official nutrient recommendations, established a recommended upper limit of 2,000 IU or 50 mcg per day of Vitamin D from all sources to help prevent overdoses.
Cancer
Up to 20 percent of cancer patients develop hypercalcemia, according to experts at Cancer Supportive Care Programs. Only rarely is cancer of the parathyroid gland itself responsible for the hormonal imbalance that results in hypercalcemia. The cancers most closely associated with hypercalcemia include lung and breast cancer and multiple myeloma (cancer of the white blood cells). These cancers spread and infiltrate the bones, and substances are released that promote the transfer of calcium from the bones into the blood. Normally, the kidneys are prepared to remove excess calcium from the blood and eliminate it from the body but in a state of hypercalcemia, the kidneys cannot keep up.
Other
Paget's disease, or any other disease that causes abnormal breakdown of bone tissue, can result in high blood calcium. Other conditions that can cause elevated blood calcium include HIV/AIDS, sarcoidosis (inflammation of body tissues, most often found in the lungs and lymph nodes), lymphoma and Addison's disease (adrenal insufficiency).
