The majority of the body phosphorus (80% to 85%) is present in the bones as hydroxyapatite. The remainder of the phosphate is present as inorganic phosphorus and phosphate esters. Calcium and phosphorus in serum usually exhibit a reciprocal relationship.
Increased serum phosphorus may occur in hypervitaminosis D, hypoparathyroidism, and renal failure. Reduced serum phosphorus levels are seen in rickets (Vitamin D deficiency), hyperparathyroidism, and Fanconi’s syndrome.
Hypophosphatemia is relatively common in hospitalized patients. Serum concentrations of phosphate between 1.5 and 2.4 mg/dL may be consider moderately decreased and are not usually associated with clinical signs and symptoms. Levels below 1.5 mg/dL may result in muscle weakness, hemolysis of red cells, coma, and bone deformity and impaired growth.
The most acute problem associated with rapid elevations of serum phosphate levels is hypocalcemia with tetany, seizures, and hypotension. Soft tissue calcification is also an important long-term effect of high phosphorus levels.