ISSN 2398-2969      



Jana Gordon

Daniel H Lewis


  • Definition: serum inorganic phosphorus <0.80 mmol/l.
  • Phosphate is a major intracellular anion. It has a structural role in cell membranes, hydroxyapatite of bone, nucleic acids and phosphoproteins including adenosine triphosphate (ATP). Important in oxygen transports (2,3-diphosphoglycerate), second messenger systems (cyclic adenosine monophosphate), coenzyme systems (nicotinamide adenine dinucleotide phosphate). Important in the regulation of protein, fat and carbohydrate metabolism. Involved in the activation of vitamin D and calcium homeostasis.
  • Phosphorus exists as organic (phospholipids, phosphate esters) and inorganic (orthophosphoric acid, pyrophosphoric acid) forms. 80-85% of total body phosphate is inorganic in bone as hydroxyapatite. 15% is organic and in soft tissues. Most serum phosphorus is inorganic orthophosphoric acid. 10-20% of inorganic is protein bound. 80-90% is free or bound to magnesium, sodium and calcium.
  • <1% of Total Body Phosphate found as extracellular active forms with only 33% of this being ionized.
  • Total Body Phosphate levels closely regulated in conjunction with calcium.



  • Redistribution into intracellular compartment:
  • Cellular energy alteration:
    • Acute liver injury increases cellular ATP demand.
    • Increased metabolic demand in sepsis (not proven).
  • Reduced intestinal absorption:
    • Vitamin D deficiency.
    • Reduced intake.
    • Administration of phosphate binders.
    • Severe malabsorptive disorders.
  • Increased renal loss:

Predisposing factors

  • See Etiology.



  • 60-70% of ingested phosphate is hydrolyzed to an inorganic form that is absorbed actively and passively. Active transport is increased by the presence of calcitriol primarily in the presence of hypophosphatemia. Passive absorption is largely dependent on the phosphorus content of the diet as well as the type of protein.
  • Phosphorus is excreted in the urine and primarily reabsorbed in the proximal tubules as a result of the effects of PTH. Increases in PTH PTH assay decreases reabsorption in the proximal tubules.
  • Other hormones that can increase reabsorption of phosphorus in the proximal tubules include growth hormone, insulin and thyroxine. ACTH, glucocorticoids and calcitonin decrease proximal tubular reabsorption of phosphorus.
  • Decreased RBC phosphorus decreases RBC, ATP and increases RBC fragility → hemolysis. Decreases RBC 2,3 DPG so impaired oxygen delivery to tissues.


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Schropp D M, Kovacic J (2007) Phosphorus and phosphate metabolism in veterinary patients. J Vet Emerg Crit Care 17 (2), 127-134 VetMedResource.

Other sources of information

  • Marino P (2007) Renal and Electrolyte Disorders: Calcium and Phosphorus. In: The ICU Book. pp 639-655. Lippincott, Williams & Wilkins.
  • DiBartola S P, Willard M D (2006) Disorders of Phosphorus: Hypophosphataemia and Hyperphosphataemia. In: Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. pp 195-209. Ed. S DiBartola, Saunders Elsevier.
  • Macintire D K, Drobatz K J, Haskins S C, Saxon W D (2005)Endocrine and Metabolic EmergenciesIn:Manual of Small Animal Emergency and Critical Care Medicine.pp 331-333. Lippincott, Williams & Wilkins.

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field

 Security code