Calcium is important in the body. It is an important part of the bone, and it exists as a charged particle called an ion in the blood and inside cells. Calcium is important to many body functions including bone formation, muscle contraction, nerve and brain function, and the release of hormones. Parathyroid hormone (PTH) and vitamin D regulate calcium balance in the body. PTH is produced by the parathyroid glands -- four small glands located in the neck behind the thyroid gland. Vitamin D is obtained from exposure of skin to sunlight, and from dietary sources such as fortified dairy products, egg yolks, fish, and fortified cereals. Primary hyperparathyroidism is the most common cause of hypercalcemia and is due to excess PTH. The drug lithium, used in treating bipolar disorder, may increase PTH release and cause hypercalcemia. A genetic mutation that affects the body's ability to regulate calcium is seen in familial hypocalciuric hypercalcemia (FHH), a benign (non-cancerous) condition. Blood calcium can also be high despite low levels of PTH, however. Some malignant tumors (for example, lung cancers, breast cancer) produce PTH-related peptide (PTHrP) which increases blood calcium. Excess vitamin D (hypervitaminosis D) from diet or inflammatory diseases can also cause hypercalcemia. Kidney failure, adrenal gland failure, hyperthyroidism, prolonged immobilization, use of a class of diuretics called thiazides, and ingestion of massive amounts of calcium (milk-alkali syndrome) are other potential causes. Hypercalcemia affects from 0.1 to 1% of the population. The widespread ability to measure blood calcium since the 1960s has improved detection of the condition, and today most patients with hypercalcemia have no symptoms. Women over the age of 50 are most likely to be hypercalcemic, usually due to primary hyperparathyroidism. | 
Endocrinology
