Factors affecting calcium metabolism in disorders of the kidney.

The ionized calcium concentration in blood is maintained within narrow limits by a complex hormonal system that includes parathyroid hormone (PTH) and vitamin D. The kidney plays a pivotal role in the physiologic action of PTH, as this peptide hormone increases tubular calcium reabsorption, decreases tubular phosphate reabsorption, and stimulates the renal 25-hydroxy-1-hydroxylase to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, the hormonal form of the vitamin that stimulates intestinal calcium absorption. Inherited and acquired disorders of tubular function and acute and chronic renal failure may disturb normal renal handling of calcium and phosphorus and the hydroxylation of 25-hydroxyvitamin D. As a result, decreased intestinal calcium absorption and hypocalcemia cause parathyroid hyperplasia and metabolic bone disease.