381 Disorders of Calcium Homeostasis

The metabolism of calcium and bone are closely linked. Ninety-nine percent of the total body calcium is present in the skeleton; the remaining 1% is labile between bone, extracellular and intracellular fluid. The total serum calcium concentrations range from 2.2 to 2.6 mmol/L. At a physiologic pH (7.4) 40% of total calcium is bound to albumin, 10% exists as complexes with bicarbonate/ phosphate/citrate, and 50% is free ionized calcium. The normal range for ionized calcium in blood plasma is 1–1.25 mmol/L. Ionized calcium has the primary regulatory role, it is in turn the regulated component. The homeostasis of calcium (Ca) is intimately linked to that of magnesium (Mg) and phosphorus. Dietary intake, fecal and urinary excretion, and bone turnover need to be balanced. Phosphorus is more widely distributed with 85% in bones as hydroxyapatite and 15% as a component of phospholipids, phosphoproteins, and nucleic acids. In blood, phosphorus exists in two ionic forms HPO4 2 and H2PO4 , together called inorganic phosphorus (Pi), and normal plasma or serum phosphorus concentration changes during life with the highest levels been found in neonates. Homeostasis of phosphorus is done mainly by the kidneys. The plasma levels of calcium and phosphorus are subject to regulation by the interaction of three hormones – parathormone (PTH), vitamin D (as 1,25dihydroxyvitamin D), and calcitonin (CT) – acting on three organs, bone, intestines, and kidneys. The influences of other hormones, notably sex steroids, growth hormone, and thyroxine and several novel gene products involved in this homeostasis are becoming clear. Pathophysiology: Disorders of calcium and bone metabolism are common and best understood by examining changes in: parathyroid hormone, 1,25dihydroxyvitamin D, urinary excretion of calcium and phosphorus, and bone turnover. Parathyroid Hormone (PTH) is an 84-amino-acid peptide secreted by parathyroid glands (usually 2 pairs) located at the back of the thyroid gland in the neck. PTH