Phosphorus

Overview

Phosphorus is the third most abundant intracellular ion:

1. Potassium

2. Magnesium

3. Phosphorus (as Phosphate)

Within the body it is found as Phosphate ions, or Phosphate within bone. The majority (~95%) is combined with Calcium in bone, forming Hydroxyapatite. Less than 1% is found in the extracelullar fluid.

Phosphorus (inorganic phosphate)

Serum phosphorus is closely related to the serum calcium concentration. It is primarily regulated through the action of parathyroid hormone and calcitonin providing a dual negative feedback control mechanism to regulate calcium homeostasis, by acting on target cells in the bone and kidney and indirectly, the intestine. Serum concentration is regulated by renal excretion under the influence of parathyroid hormone. In patients with decreased glomerular filtration rate concentrations will rise due to reduced clearance. Abnormal levels are caused by variations in dietary intake, decreased renal excretion, drug therapy (diuretics, salicylates and phosphate containing enemas) and the hormonal imbalances that affect serum calcium. Translocation between the intracellular and extracellular compartment is governed by similar factors to potassium, hence an intracellular shift is promoted by insulin, alkalosis and glucose infusion. References: NationWide Laboratories

Homeostasis

Serum concentrations of phosphorus are maintained by dietary sources. It is absorbed across the intestine via passive diffusion and vitamin D mediated active transport. Phosphorus is excreted in the urine or saliva (particularly in ruminants).

Hormonal Regulation

PTH

PTH mobilises phosphorus from bone, acting to increase the plasma concentration of P. It increases phosphorus loss in the urine and saliva, so ultimately, PTH acts to remove phosphorus from the body entirely.

Calcitriol

Calcitriol increases phosphorus absorption from the intestines.

Pathology

Diseases relating to phosphorus homeostasis include:

1. Primary and Secondary Hyperparathyroidism

2. Hyperphosphataemia

3. Hypophosphatemia

Causes of hyperphosphataemia

Small Animals

• Prerenal, renal and post renal azotemia
• Young animals (dogs <1yr, cats <6mth)
• Feline hyperthyroidism
• Primary hypoparathyroidism
• Hypervitaminosis D
• Acute tumour lysis syndrome
• Osteolytic lesions including neoplasia

Equine

Serum phosphate levels fall readily after exercise to half the resting concentration and may take 3 hours to return to normal. Therefore samples should be taken at rest. A decrease may also occur during carbohydrate utilisation after feeding.

• Following general anaesthesia
• Acute renal failure
• Endurance exercise
• Normal in young animals
• Excess vitamin D
• Nutritional secondary hyperparathyroidism
• Hypoparathyroidism (rare)

Causes of hypophosphataemia

Small Animals

• Increased PTH or PTHrP: Hypercalcaemia of malignancy and Primary hyperparathyroidism
• Hyperinsulinism (endogenous/exogenous)
• Prolonged diuresis
• Fanconi syndrome
• Canine eclampsia

Equine

• Chronic renal insufficiency
• Severe scouring
• Primary hyperparathyroidism (rare)
• Vitamin D deficiency (rare)

Complementary tests

Small Animals

Calcium, urea and creatinine, urine specific gravity, T4, PTH, PTHrP.

Equine

Urea, creatinine, urine SG and urine clearance ratios.

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References

Inorganic Phosphate: NationWide Laboratories

Causes of hyperphosphataemia: NationWide Laboratories

Causes of hypophosphataemia: NationWide Laboratories

Complementary tests: NationWide Laboratories

Phosphorus Learning Resources
Flashcards Test your knowledge using flashcard type questions	Phosphorus homeostasis.

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