What is hyperphosphatemia?
Hyperphosphatemia is the term for elevated phosphates in the blood. Phosphates are the naturally occurring form of phosphorus and is necessary for many processes in the body. Most of the phosphates within the body are found in the bones and the concentration in the bloodstream is carefully regulated. Excess may be excreted through the kidneys. Hyperphosphatemia arises when excess phosphates are absorbed from the gut, reduced excretion of phosphates and higher than normal production of phosphates.
Phosphates in the Human Body
Phosphates are essential for a number of different processes in the human body. Most of the phosphates are in the bones in the body forming part of the mineralized matrix where it contributes to the strength of the skeletal system. Small amounts of phosphates enter and exit the bones daily. Apart from the bones, phosphates are also found within every cell and the amount in the bloodstream is limited under normal circumstances. Phosphate is also an important component of nucleic acids – DNA and RNA. It is also a part of ATP which is needed for energy by every cell in the human body.
Unlike other chemicals, such as potassium and sodium, a sudden and excessive increase in the phosphate levels in the blood do not have as detrimental an effect. Nevertheless, it is important that the phosphate levels in the blood are carefully regulated for homeostasis – the delicate balance that is essential for health. It is important to understand how the phosphate levels are regulated in the bloodstream to further understand how hyperphosphatemia may arise. The balance is maintained by the intake of phosphates, excretion through the kidneys and the shift of phosphate into the blood and tissue fluid.
Causes of High Phosphate Levels
Phosphate levels rise when there is :
- Excessive intake of phosphate.
- Decreased excretion of phosphate.
- Movement of phosphate from inside cells to the blood and tissue fluids.
Excessive Phosphate Intake
Phosphate is absorbed from digested food in the gut. Normally the amount of phosphate in the blood is kept in check by excretion of excess phosphate through the kidneys. Even if there is higher than normal phosphate absorption, the kidneys are able to cope with increased excretion. However, should the kidney function be hampered, even a moderately raised phosphate absorption can lead to hyperphosphatemia. Phosphate absorption can also be increased with vitamin D. Therefore vitamin D toxicity can lead to hyperphosphatemia through increased gut absorption of phosphate. In addition, vitamin D increase reabsorption of phosphate from the kidney tubules further contributing to phosphate retention.
Some of the possible causes of hyperphosphatemia due to excessive phosphate intake includes :
- Intravenous injection of phosphate
- Excessive intake of vitamin D
- Enemas with saline phosphate solutions
- Acute phosphorus poisoning
- Milk-alkali syndrome
Decreased Excretion of Phosphate
If the elimination of phosphate via the kidneys are diminished, especially when there is high intake of phosphate in foods, hyperphosphatemia may occur. This can be seen with kidney problems such as acute or chronic renal failure. Parathyroid hormone (PTH) secreted by the parathyroid gland also contributes to regulation of phosphate. By acting on the kidney, PTH helps with the excretion of phosphate from the blood. However, when PTH levels are low, then there is increased reabsorption of phosphate leading to retention. Therefore hyperphosphatemia may arise with decreased PTH levels – hypoparathyroidism. As explained above, hyphosphatemia can also occur with high vitamin D levels as vitamin D increases phosphate reabsorption from the kidney thereby decreasing phosphate excretion.
Some of the possible causes of hyperphosphatemia due to decreased elimination includes :
- Kidney failure
- Magnesium deficiency
- Excessive intake of vitamin D
- Use of bisphosphonates for osteoporosis treatment
- Elevated blood triglycerides (hypertriglyceridemia)
- Multiple myeloma
Movement of Phosphate into the Blood
Most of the phosphate is contained within the bones and the rest is within the cells of the body. Comparatively, only small amounts are in the circulation at any time. However, with certain disturbances in the body there can be a shift of phosphate out of the tissues and into the body fluids, including the blood. This shift arises with various medical conditions like with cancer treatments (tumor lysis syndrome), muscle breakdown (rhabdomyolysis), increased body fluid acidity (acidosis) or deficiency of the hormone insulin. This shift in phosphate is an uncommon cause of hyperphosphatemia but can exacerbate the condition due to one of the other mechanisms discussed above.
Signs and Symptoms
Hyperphosphatemia is largely asymptomatic in most patients. Non-specific symptoms include :
- Pain in joints and bones
- Itchy skin (pruritus) even without a rash
- Skin rashes which are usually itchy
These symptoms do not arise immediately but develop with time. The more prominent clinical features seen with hyperphosphatemia is instead due to the effects of high phosphate load on the blood calcium levels. It leads to low blood calcium (hypocalcemia) and it is these symptoms that may be more prominent. This includes :
- Low blood pressure
- Muscle cramps
- Numbness and tingling
- Abnormal sweating
- Breathing difficulty
- Abdominal pain
In addition, the other symptoms may be seen are a result of the underlying cause of hyperphosphatemia.
Tests and Diagnosis
Hyperphosphatemia is diagnosed with specific blood tests as the clinical presentation is vague. These tests measure the levels of :
- Blood urea nitrogen (BUN)
- Vitamin D
- Parathyroid hormone (PTH)
Urine tests are not usually necessary.
Treatment for Hyperphosphatemia
The underlying cause of hyperphosphatemia should be diagnosed and treated in order to restore normal phospate metabolism Various medication can help normalize the phosphate levels in the blood. Dietary modification is also necessary to minimize high phosphate intake. The medication used for hyperphosphatemia includes :
- Phosphate binders which blocks the absorption of phosphates from the gut. This includes :
- Aluminum hydroxide
- Calcium carbonate
- Calcium acetate
- Magnesium hydroxide
- Sevelamer hydrochloride
- Lanthanum carbonate
- Loop diuretics increase phosphate excretion through the kidneys. These drugs include :
High Phosphate Foods to Avoid
A low phosphate diet is an important part of the treatment and management of hyperphosphatemia. Dietary change alone may be sufficient to restore blood phosphate levels provided that the kidneys are still functioning fairly efficiently. Foods that should be avoided, or at least consumed in moderation includes :
- Dairy :
- Ice cream
- Cheese (harder varieties should be avoided while cottage or cream cheese can be consumed)
- Dark cola drinks
- Fish, certain species like pollock, sardines, walleye.
- Organ meats (offals)
- Soups containing large amounts of beans, lentils, peas and milk which are high in phosphorus.
- Vegetables and legumes :
- Sweet potato
- Whole grains including whole grain food products.