Hypercalcemia results when the movement of calcium into the circulation overwhelms the ability of the regulatory hormones or the renal system to eliminate excess calcium ions. Hypercalcemia is documented when the serum calcium level rises above 10.5 mg/ dL or 5.5 mEq/L. Ionized serum calcium levels must rise above 5.25 mg/dL or 2.5 mEq/L for hypercalcemia to exist.

Hyperparathyroidism can be responsible for hypercalcemia due to the increased production of parathyroid hormone, which helps activate calcium from bone (Table 18-1). Another cause for hypercalcemia is the destruction of bone secondary to malignant cells of certain cancers, such
as lung, breast, ovary, prostate, gastrointestinal, and leukemia. Often, the symptoms of hypercalcemia mimic those of the malignancy. The patient seeks medical attention for the symptoms of the undiagnosed malignancy and at the same time receives the diagnosis of hypercalcemia.