What is the most likely cause of a serum calcium level of 11.6 mg/dL and elevated parathyroid hormone levels?

A serum calcium level of 11.6 mg/dL, which indicates hypercalcemia, alongside elevated parathyroid hormone (PTH) levels is highly suggestive of primary hyperparathyroidism. In this condition, the parathyroid glands are overactive, leading to increased secretion of PTH, which in turn raises the serum calcium level by increasing calcium absorption from the intestine, enhancing renal tubular reabsorption of calcium, and mobilizing calcium from the bones.

In cases of primary hyperparathyroidism, it’s common to find elevated calcium levels and elevated PTH simultaneously, as the glands are responding inappropriately despite the high calcium levels. This contrasts with secondary hyperparathyroidism, where PTH is elevated due to a low serum calcium level, often in response to other conditions like renal failure or vitamin D deficiency.

Vitamin D deficiency would typically lead to low serum calcium and compensatory elevated PTH levels, which does not fit this scenario. Familial hypocalciuric hypercalcemia is a genetic condition that usually presents with elevated calcium levels but is characterized by low or inappropriately normal PTH levels due to a defect in calcium sensing.

Thus, in this case, the combination of elevated calcium and elevated P