Hypercalcemia

Hypercalcemia means there's too much calcium in your blood. Often there's no clue at all: it usually turns up on a routine blood test, and most people don't have any symptoms.

To see why calcium goes up, it helps to meet what controls it: parathyroid hormone, or PTH, which is made by the parathyroid glands. Picture the calcium in your blood as the water in a bathtub, with PTH working the faucets that fill it. One faucet comes from bone, where most of the body's calcium is stored. PTH increases bone resorption, which means it breaks down a little bone so it releases its calcium. Another faucet comes from the kidneys. Through renal reabsorption, the kidneys take calcium back and return it to the blood instead of letting it go. The last one comes from the intestine: PTH helps make more active vitamin D, and that vitamin D gets the intestine to absorb more calcium.

If those faucets stay open too wide, the tub overflows. That's why the most common cause of hypercalcemia is too much PTH released by the parathyroid glands. In cancer, something similar usually happens by a different route: the tumor makes a protein related to PTH that acts on the hormone's receptors, the spots on cells where PTH latches on to deliver its message.