Pathophysiology of Hyperparathyroidism Parathyroid hormone (PTH) increases serum calcium by Enhancing distal tubular calcium reabsorption Rapidly mobilizing calcium and phosphate from bone (bone resorption) Increasing intestinal absorption of calcium by stimulating conversion of vitamin D to its most active form, calcitriol Hyperparathyroidism is characterized as Primary: Excessive secretion of PTH due to a disorder of the parathyroid glands Secondary: Hypocalcemia due to non-parathyroid disorders leads to chronic PTH hypersecretion Tertiary: Autonomous secretion of PTH unrelated to serum calcium concentration in patients with long-standing secondary hyperparathyroidism Primary hyperparathyroidism Primary hyperparathyroidism is a generalized disorder resulting from excessive secretion of parathyroid hormone by one or more parathyroid glands.