PTH and primary hyperparathyroidism: Although hypercalcemia most often results from either primary hyperparathyroidism or malignancy, any disease of these organs may lead to dysregulation of calcium balance and variations in serum calcium.1 Parathyroid hormone (PTH) tightly regulates serum calcium through multiple mechanisms: PTH increases renal 1,25(OH)2D synthesis to promote intestinal calcium absorption, enhances renal reabsorption of calcium in the distal tubules, and increases bone resorption to maintain normocalcemia.