Hypercalcemia is a less common disorder than hypocalcemia, occurring in about 0.6–7.5% of hospitalized and less than 1.0% of outpatients [1–6].

Hypercalcemia is defined as total serum calcium >10.5 mg/dL (>2.6 mmol/L) with normal serum albumin or ionized calcium >5.2 mg/dL (>1.3 mmol/L) [7].

When total serum calcium is >14.0 mg/dL (>3.5 mmol/L) or ionized calcium is >7.0 mg/dL (>1.7 mmol/L), it is considered severe hypercalcemia [7]. Early detection and prompt treatment of hypercalcemia are essential because it carries high morbidity and mortality [7, 8].

Primary hyperparathyroidism and malignancy are the two most common causes of hypercalcemia in more than 90% of patients [7, 9]. In recent times, there has been a significant rise in hypercalcemia due to Vitamin D toxicity.

Mechanisms by which different etiologies cause hypercalcemia are enhanced bone resorption, increased intestinal absorption, or decreased renal calcium excretion (Table 25.1).

This disorder is the most common cause of hypercalcemia in about 50% to 60% of ambulatory and 25% of hospitalized patients [10]. Primary hyperparathyroidism occurs in middle age (between 50 and 60 years) and is two to three times more common in women. It is characterized by an elevated PTH, hypercalcemia, hypophosphatemia, loss of cortical bone, and hypercalciuria. Hypercalcemia in this condition is usually mild, and in >80% of patients, it occurs due to a single parathyroid adenoma [7]. An abnormal, incompletely-regulated, high secretion of parathyroid hormone (PTH) from the parathyroid gland activates osteoclast, causing bone resorption and increasing intestinal calcium absorption, leading to hypercalcemia [11].

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