Hypercalcaemia occurs when the complex homeostatic mechanism that is responsible for maintaining normal extracellular fluid calcium concentrations is overwhelmed. It usually involves an increase in entry of calcium into the extracellular fluid from two of the major organ systems that control calcium homeostasis. In primary hyperparathyroidism there is an increase in renal tubular calcium reabsorption, an increase in bone resorption, and an increase in calcium absorption from the gastrointestinal tract. There is a marked increase in bone resorption in many patients with malignant disease, together with an increase in renal tubular calcium reabsorption.
