Phosphate, PTH and Kidney Disease

What is Phosphate?

image courtesy of wikipedia.

Phosphate is an inorganic ion which is present in many foods and preservatives of food. Here is a list of foods that are high in phosphate.

What are the Disorders of Phosphate Metabolism.

1. Hyperphosphatemia

2. Hypophosphatemia.

1. Hyperphosphatemia.




Elevated levels of phosphate are common in patients with kidney disease due to the disturbance of the bodies natural method of controlling phosphate levels, the kidney.
The kidney is responsible for eliminating phosphate from the body based on the concentration of phosphate in the blood and the concentration of a hormone known as parathyroid hormone.



When the kidney is unable to get rid of the phosphate within the body the levels of phosphate begin to rise and this stimulates the parathyroid glands, these glands are located in the neck behind the thyroid. The contain special sensors that inform the gland about the levels of calcium, vitamin d and phosphate within the blood.

In response to increased levels of phosphate parathyroid hormone or PTH is secreted. This hormone stimulates bone to take up calcium and phosphate when vitamin d is present in sufficient quantities thus decreasing the blood level of phosphate. It also stimulates the kidney to increase excretion of phosphate in the urine. The combination of bone uptake and urine excretion will regulate phosphate.

If the kidney is unable to secrete phosphate despite the stimulation of PTH the result will be persistently elevated levels of both phosphate and PTH.

2. Hypophosphatemia.




Hypophosphatemia is much less common than hyperphosphatemia.

Causes include:-

• In patients with malnutrition levels of phosphate may be low. They may go even lower when patients are subsequently fed.

• When the blood becomes alkalotic a state that can occur with intoxication of certain drugs or in sever illness phosphate levels in the blood fall.

• Alcohol impairs phosphate absorption. Alcoholics are also malnourished with regard to minerals.

• Abnormalities of the gut which prevent the absorption of phosphate.

• Defects of the kidney or its regulation that leads to excessive excretion of phosphate in the urine.

Why are High Phosphate Levels so Bad?

• Studies have shown that high phosphate levels are linked to increased risk of death from cardiovascular disease.

• It has also been shown that increased levels of phosphate is associated with deposition of calcium phosphate crystals in the skin a condition known as uremic calciphylaxis. This is a debilitating process that leads to loss of skin, it is very difficult to treat and has a high rate of mortality once diagnosed. Prevention here is much better than cure.

• High phosphate levels occur with increased levels of PTH this results in continuous activation of bone metabolism this leads to a disease of the bone known as renal osteodystrophy. Where the bones can become painful and brittle prone to having fractures.

• High PTH levels as a result of phosphate will also affect the marrow of the bone causing worsening of anemia. Anemia in CKD is usually treated by prescribing EPO. If PTH levels are high for long enough the bone marrow may be damaged and not be able to respond to EPO.



Structure of the PTH protein. Based on PyMOL rendering of PDB 1bwx. (Photo credit: Wikipedia)

• High phosphate levels are associated with more pain in general in ESRD and a lower quality of life.

What is the Treatment for High Phosphate Levels.

Decrease the amount of phosphate entering the body and increase the amount of phosphate leaving the body.

• Decrease the phosphate by having a low phosphate renal diet.

• Decrease the phosphate entering the body by binding it in the gut thus preventing absorption by using.

Phosphate binders such as:-

• TUMS (calcium carbonate)

• PHOSLO (calcium acetate)

• RENAGEL (sevelamer).

photo credit: trekkyandy

Increase the amount of phosphate leaving the body.

• This can be accomplished via dialysis, however conventional dialysis is notoriously poor for removing phosphate. The removal of phosphate is linked to length of time spent on dialysis therefore longer treatment is more effective for phosphate removal.

• Some of the effects of high phosphate are believed to be due to elevated levels of PTH. Therefore as a means to improving outcomes in patients with high phosphate and high PTH, one or more parathyroid glands may be removed to reduce the total amount of PTH circulating in the blood.

• An alternative to surgical removal of the gland is a drug known as cinacalcet which directly turns of PTH production by the parathyoid.

• Such drastic measures maybe necessary in patients that have persistently elevated levels of both phosphate calcium and PTH despite other therapies.