When a patient develops hypercalcemia, a disorder of the parathyroid glands or malignancy at a visceral location must be considered. The elevated parathyroid hormone (PTH) suggests primary hyperparathyroidism. The most common cause of primary hyperparathyroidism is a parathyroid adenoma. Secondary hyperparathyroidism, most commonly resulting from renal failure, is excluded when the serum inorganic phosphate level is low because phosphate is retained with chronic renal failure. Hypervitaminosis D can cause hypercalcemia because of increased calcium absorption, but in these cases, the PTH levels are expected to be near the low end of the reference range because of feedback suppression. Serum PTH levels near the high end of the reference range indicate autonomous PTH secretion unregulated by hypercalcemia. Although medullary carcinomas of the thyroid often have positive immunohistochemical staining for calcitonin, and plasma levels are sometimes increased, there is typically no major reduction in serum calcium as a result.