What is the most common cause of hypercalcemia in hospitalized patients?

The most common cause of hypercalcemia in hospitalized patients is malignancy. Various malignancies can lead to hypercalcemia through different mechanisms, including osteolytic metastasis, where cancer spreads to bone and causes bone resorption, or by producing parathyroid hormone-related peptide (PTHrP), which mimics the action of parathyroid hormone and increases blood calcium levels. Conditions such as squamous cell carcinoma of the lung, breast cancer, and multiple myeloma are particularly notable for being associated with hypercalcemia.

While dehydration can contribute to hypercalcemia by causing concentration of serum calcium due to reduced plasma volume, it is not the most common cause in hospitalized patients. Similarly, primary hyperparathyroidism and thyroid disease can cause hypercalcemia, but they are less prevalent causes in a hospitalized setting, where cancer is more often encountered as a source of elevated calcium levels. Thus, the association of malignancy with hypercalcemia makes it the most relevant choice in this context.