Why Is My Dog Or Cat’s Blood Phosphorus Level High or Low?

Ron Hines DVM PhD

Your Pet’s Blood Phosphorus Level = Phosphate, PHOS

See What Normal Blood & Urine Values Are

Causes Of Most Abnormal Blood & Urine Tests

See How Tests Are Grouped

Most of your dog and cat’s phosphorus is locked away in its bones (85%) as phosphate. In its bones phosphorus combines with oxygen and calcium to form calcium phosphate (hydroxyapatite). But a small portion of the phosphorus circulates in your pet’s blood as free phosphate ion (PO4). 

That portion of phosphorus/phosphate is very important for proper nerve function and muscle contraction. Your pet’s kidneys – under the control of parathyroid gland hormone (PTH) – direct your dog or cat’s kidneys to allow excess phosphate into the urine to help keep circulating phosphate level from rising too high. When your pet’s blood phosphate level is abnormally high, failing kidneys are the most common medical cause.

Calcium and phosphate circulate in your pet’s blood in a teeter-totter relationship. When one is higher than normal, the other is almost always low. Veterinary laboratories and in-office analytical machines almost always determined both at the same time. When your veterinarians notes on the lab report that your dog or cat’s blood phosphorus level is high, the next values he/she is likely to look at are your pet’s BUN and Creatinine levels. All three being high generally confirms a late-stage kidney problem. 

Reasons Why Your Pet’s Blood Phosphorus Level Could Be High (=Hyperphosphatemia):

I listed these causes in the order of frequency with which they are likely to occur:

Blood Sample Hemolysis

There is a considerable amount of phosphorus in the red blood cells (RBCs =erythrocytes) that circulate in your dog and cat’s blood. If those RBCs are accidentally broken during collection, your pet’s blood sample will give a falsely high blood phosphate reading.  Possible causes are the collection of the blood sample through too small a needle, too much suction on the syringe or too slow a blood flow into the syringe. Those things happen. Waiting too long before separating blood serum from the RBCs can also rupture (lyse) the RBCs and release phosphorus. 

Kidney Disease

I mentioned that the most common cause of elevated blood phosphorus (particularly in middle-aged to elderly dogs and cats) is impending kidney failure. Every life has its weakest link – the first organ to fail due to the wear and tear of time. Until recently in humans, it was our hearts. (read here) In dogs and cats it is often their kidneys that wear out first. Blood phosphorus levels increases late in progressive kidney failure with the phosphorus increase proportional to the degree of kidney damage.

Your pet’s kidney’s ability to remove excess phosphate is dependent on adequate blood flow through the kidneys. So, heart issues and other health challenges that decrease blood flow through the kidneys (= decreased GFR) can also elevate your dog and cat’s blood phosphate level.

When blood phosphorus levels go up in younger dogs and cats, think of things like inherited genetic defects (e.g. polycystic kidney disease), a blocked urinary tract (e.g. kidney stones), kidney infections (e.g. leptospirosis).

Kidney-toxic ingestions of things like antifreeze, certain vitamin D-containing rodent poisons and other toxic products need to be ruled out. Xylitol, a sugar substitute, when consumed by dogs can initially cause blood phosphate levels to drop below normal. But with time, the kidney damage xylitol causes can cause phosphorus levels to exceed normal levels. 

Hyperthyroidism (usually in a cat) can also cause blood phosphate levels to increase.

Parathyroid gland disease that leads to lower levels of PTH hormone or results from damage to those glands during neck surgery often elevates blood phosphorus level. (ref)

Young growing animals often have slightly higher blood phosphate levels than what would be considered normal when they are adults. Your dog or cat’s blood phosphate level can show a slight increase after a recent meal. 

Phosphate is also occasionally elevated when a bone tumor (osteosarcoma) is present.

Monoclonal gammaopathy, cancer of the bone marrow, can also be associated with high blood phosphate levels. 

Phosphorus blood levels also occasionally rise in pet’s (cats) whose pituitary gland secretes too much growth hormone (=acromegaly).

Acidosis, usually as the result of uncontrolled diabetes (diabetic ketoacidosis), can also raise your dog or cat’s blood phosphate levels.

Fleet™-type phosphate enemas, particularly when the full human dose is given to a small pet, can raise blood phosphate to very dangerous levels. Some urinary acidifiers also contain phosphate. 

Tissue and muscle destruction (rhabdomyolysis and similar myopathies) or from a traumatic event can cause a temporary rise in your pet’s blood phosphate level.  

Starvation, sustained vomiting and diarrhea have all been associated with both high and low blood phosphate levels.  So might homemade diets with unusually high phosphorus content. 

Too much vitamin D, as might be seen in ingestion of certain rodent poisons, excessive multivitamin supplementation, or accidental addition of excessive vitamin D-3 to a batch of commercially purchased diets have all been associated with unusually high blood phosphorus level. 

Reasons Why Your Pet’s Blood Phosphorus Levels Might Be Too Low (=Hypophosphatemia):

A low blood phosphorus level is considerably less common than an elevated blood phosphorus level. The causes are also less well understood.

Anything that causes your pet’s blood calcium level to go up will probably cause its blood phosphorus level to go down. That can include a large recent meal, the use of phosphate binding medications (e.g. aluminum hydroxide, sucralfate), corticosteroid medications or Cushing’s disease.

Certain tumors that release PTH-related peptides, feeding very low phosphorus or low vitamin D diets, insulin or glucose injections and the diuretic, furosemide (Lasix®) have all been associated with low blood phosphate.

Low body temperature (hypothermia), hepatic lipidosis in cats, and some genetic kidney defects have also been reported to cause low blood phosphate levels.

High levels of bilirubin in your dog or cat’s blood sample or the use of anticoagulants in the collection tube (needs to be a red top tube) can also cause falsely low blood phosphate results.

Complementary Tests:

CBC /  WBC and blood chemistry panel especially for elevations in BUN and creatinine,   urinalysis with urine specific gravity (looking for the low SpGr of chronic kidney disease),   Free T4 levels for evidence of hyperthyroidism, blood PTH levels for evidence of hyperparathyroidism  (not hyperthyroidism)  cats. A review of your dog and cat’s diet for possible nutritional deficiencies


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