In the body, almost all phosphorus is combined with oxygen, forming phosphate. Bone contains about 85% of the body's phosphate. The rest is located primarily inside cells, where it is involved in energy production.
Phosphate is necessary for the formation of bone and teeth. Phosphate is also used as a building block for several important substances, including those used by the cell for energy, cell membranes, and DNA (deoxyribonucleic acid). The body obtains phosphate from foods and excretes it in urine and stool.
In hypophosphatemia, the level of phosphate in blood is too low.
The phosphate level in blood may become low over time, resulting in chronic hypophosphatemia. Chronic hypophosphatemia usually develops because too much phosphate is excreted. Causes include the following:
The phosphate level in blood can suddenly fall dangerously low in people recovering from the following conditions because the body uses large amounts of phosphate during recovery:
This may result in an irregular heart rhythm and even death.
Symptoms occur only when the phosphate level in blood becomes very low. Muscle weakness develops, followed by stupor, coma, and death. In mild chronic hypophosphatemia, the bones can weaken, resulting in bone pain and fractures. People may become weak and lose their appetite.
Diagnosis and Treatment
The diagnosis is based on blood tests indicating that the phosphate level is low. Doctors do other tests to identify the cause if it is not readily apparent.
Any drugs that can reduce the phosphate level are stopped. If hypophosphatemia is mild and causes no symptoms, drinking low-fat or skim milk, which provides a large amount of phosphate, may help. Or people can take phosphate by mouth, but doing so usually causes diarrhea. If hypophosphatemia is very severe or if phosphate cannot be taken by mouth, phosphate may be given intravenously.
In hyperphosphatemia, the level of phosphate in blood is too high.
Hyperphosphatemia is rare except in people with severe kidney dysfunction. In these people, the kidneys do not excrete enough phosphate. Dialysis, often used to treat them, is not very effective at removing phosphate and thus does not reduce the risk of hyperphosphatemia.
Less commonly, hyperphosphatemia develops in people with the following:
Most people with hyperphosphatemia do not have symptoms. However, in people with severe kidney dysfunction, calcium combines with phosphate to form crystals (calcify) in the walls of the blood vessels and heart. Severe arteriosclerosis (hardening of the arteries) can result, leading to strokes, heart attacks, and poor circulation. Crystals can also form in the skin, where they cause severe itching.
Diagnosis and Treatment
The diagnosis is based on blood tests indicating that the phosphate level is high.
Hyperphosphatemia in people with kidney dysfunction is treated by reducing consumption of phosphate and reducing absorption of phosphate from the digestive tract. Foods that are high in phosphate should be avoided. Drugs that bind with phosphate, such as sevelamer and calcium compounds, should be taken with meals as prescribed by a doctor. By binding with phosphate, these drugs make it harder to absorb, and more phosphate is excreted. Sevelamer is often used for people undergoing dialysis because calcium compounds can make calcium-phosphate crystals more likely to form in tissues.
Last full review/revision August 2008 by Larry E. Johnson, MD, PhD