Interpreting and Correlating Abnormal Laboratory Values : Electrolyte Abnormalities

Electrolyte Abnormalities

Hyponatremia

The four most common causes of hyponatremia are given in Table 1 (See below), together with a fifth, rare, cause, Bartter's syndrome. A sixth, metabolic cause, diabetes mellitus, is also presented in this table. In all forms of hyponatremia, the chloride ion concentration is also generally low since chloride is the chief counter-ion for sodium.

HORMONES REGULATING MINERAL METABOLISM

REGULATION OF MINERAL METABOLISM:

PTH and 1, 25-dihydroxyvitamin D are the primary hormones regulating bone and mineral metabolism. Physiological role of calcitonin has not yet been established. PTHrP is the principal mediator of humoral hypercalcemia of malignancy.

PARATHYROID HORMONE

PTH (chromosome 11) is synthesized and secreted by parathyroid glands located posterior to thyroid gland. The glands consist of chief and oxyphil cells; the chief cells synthesize, store, and secrete PTH. It is cleared by liver and kidney. PTH acts directly on bone and kidney, and indirectly on intestine to regulate concentration of calcium and phosphate in plasma.

Phosphate: Introduction, Clinical significance and measurement

An adult contains 600 g of phosphorus in inorganic and organic phosphates, of which about 85% is in skeleton and rest in soft tissue and extracellular fluid. Though plasma has both organic and inorganic phosphate but inorganic phosphate (H₂PO₄^- and HPO₄^2-) is measured. Approximately 10% is protein bound and 35% complexed with sodium, calcium, magnesium; and remainder 55% is free. The organic phosphates are located within the cell of blood.

Inorganic phosphate is a major component of hydroxyapatite in bone and is the source of intracellular and extracellular pool. Organic phosphate in cells is found to be incorporated into nucleic acid, phospholipids, phosphoproteins and ATP, GTP, Creatine phosphate, etc. Phosphate is important for activity of adenylate cyclase, 25-hydroxy vitamin D-1α-hydroxylase and those involved in 2, 3-diphosphoglycerate.

Calcium : Introduction, clinical significance and measurement

About 99% of calcium occurs in bone, along with calcium 85% of phosphate and 55% magnesium. The concentration of these minerals depends on net effect of bone mineralization, intestinal absorption and renal excretion. PTH and 1, 25-dihydroxyvitamin D are the principle hormones regulating these three processes.