How can paraneoplastic hypercalcemia be distinguished from primary hyperparathyroidism?

Paraneoplastic hypercalcemia is typically characterized by normal or decreased parathyroid hormone (PTH) levels, as the hypercalcemia results primarily from the secretion of parathyroid hormone-related peptide (PTHrP) or other factors by tumors. In contrast, primary hyperparathyroidism is associated with elevated PTH levels, as the hypercalcemia is due to an excess of PTH, usually caused by a parathyroid adenoma or hyperplasia.

This distinction is crucial in clinical practice because the management of these two conditions differs significantly. In paraneoplastic hypercalcemia, treatment focuses on the underlying malignancy and may include hydration, bisphosphonates, or other agents to lower calcium levels. In contrast, primary hyperparathyroidism may require surgical intervention to remove the overactive parathyroid tissue.

Therefore, measuring PTH levels is a key diagnostic criterion that can effectively differentiate between paraneoplastic hypercalcemia and primary hyperparathyroidism, making the identification of normal or decreased PTH levels essential in establishing the correct diagnosis.