Which test would help differentiate between primary and secondary hypercalcemia?

The most effective test to differentiate between primary and secondary hypercalcemia is the measurement of intact parathyroid hormone (PTH) levels. In primary hypercalcemia, which is often caused by primary hyperparathyroidism, PTH levels are usually elevated due to the hyperactivity of the parathyroid glands. Conversely, in secondary hypercalcemia, which typically occurs in conditions like chronic kidney disease or malignancy, PTH levels can be normal or elevated as the body tries to compensate for the underlying issues affecting calcium metabolism. Thus, analyzing intact PTH levels provides valuable insight into the underlying cause of hypercalcemia, aiding clinicians in establishing a diagnosis and determining the appropriate management plan.