Secondary hyperparathyroidism occurs most commonly secondary to chronic renal failure. Other causes of secondary hyperparathyroidism include osteomalacia, rickets, and malabsorption. The pathophysiology of secondary hyperparathyroidism results from abnormalities in the renal tubular absorption of phosphate with reduced phosphate excretion and hyperphosphatemia, as well as impaired renal conversion of 25-hydroxycholecalciferal to 1,25-dihydroxycholecalciferol, which in turn leads to a decrease in the intestinal absorption of calcium. In combination, elevated serum phosphate levels and reduced vitamin D production result in decreases in serum calcium levels or hypocalcemia, leading to hyperparathyroidism.