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.

Small animals

Causes of hyperphosphataemia

• 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

Causes of hypophosphataemia

• Increased PTH or PTHrP

Hypercalcaemia of malignancy

Primary hyperparathyroidism

• Hyperinsulinism (endogenous/exogenous)
• Prolonged diuresis
• Fanconi syndrome
• Canine eclampsia

Complementary tests

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

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.

Causes of hyperphosphataemia

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

Causes of hypophosphataemia

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

Complementary tests

Urea, creatinine, urine SG and urine clearance ratios.

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Authors & References

NationWide Laboratories