Most people with kidney failure have some degree of renal bone
disease. This is because one of the functions of the kidneys is
to help make the bones strong and healthy. For strong bones, the
kidneys must be able to maintain a healthy balance of various
substances such as: calcium, phosphate and vitamin D in the
body. Kidney failure results in abnormal levels of these
substances, and so leads to renal bone disease.
Development of renal bone disease
Blood tests will reveal abnormal levels of calcium, phosphate
and vitamin D very early in patients with kidney failure. The
calcium and vitamin D levels will be too low, while the
phosphate level will be too high. Normally the calcium and
vitamin D levels are not noticed by the patient until after the
start of dialysis, but high phosphate levels causes the patient
to have itchy skin. Treatment should be started at an early
stage to prevent weakening of the bones. Without treatment,
renal bone disease can lead to pain in the bones, especially in
the back, hips, legs and knees. The weakened bones also become
increasingly prone to fracture. Fortunately, early recognition
and treatment of renal bone disease means that bone pain and
fractures are now uncommon in kidney patients.
Causes of renal bone disease
Most kidney patients will have a combination of the following
causes of renal bone disease:
1. Low vitamin D
levels in blood - Vitamin D helps the body absorb calcium
from foods to strengthen bones. It is found in some foods such
as margarine and butter and mostly made by the skin (a process
which only occurs if stimulated by sunlight). Unfortunately,
vitamin D found in food and made by the skin cannot be used
directly by the body and requires the kidneys to transform
vitamin D into the active substance.
2. High phosphate
levels in the blood - Foods that contain phosphate
include dairy products, nuts and meat. Phosphate is also stored
in the body in the bones and also present in the blood. Normally
kidneys help to keep the right amount of phosphate, but in
kidney failure, phosphate builds up in the blood. Normal
phosphate levels in the blood are 0.8 to 1.4mmol/l. In kidney
patients phosphate levels are high-rising to more than
2.0mmol/l. Unfortunately phosphates are not easily removed by
dialysis and it is, therefore, important to watch your diet to
make sure the phosphate levels do not rise too high. High levels
can cause severe itching which is uncomfortable.
3. Low calcium levels in the blood -
Calcium is stored in our bones and is obtained from foods,
especially dairy products, eggs and green vegetables. We also
have calcium in our blood. The kidneys normally help to keep
calcium in the bones by producing the active form of vitamin D
and controlling phosphate levels. Phosphate and calcium levels
are controlled together in the blood and tend to act like a
see-saw - if one rises then the other falls. A high phosphate
level in renal failure tends to lead to a low calcium level.
Therefore in kidney failure the calcium drains out of the bones
and is lost from the body. This leads to a fall in the level of
calcium in the blood. Normal calcium blood levels are between
2.2 and 2.6 mmol/l. In kidney patients the level of calcium in
the blood may fall below 2.0 mmol/l.
hormone and kidney failure - Parathyroid Hormone (PTH) is
a substance produced by four tiny glands called the parathyroid
glands, which are situated in the front of the neck. The PTH
helps to control blood levels of calcium.In kidney failure, the
patient's blood calcium can become low. If this happens the PTH
can boost it by causing calcium to drain from the bones. Raised
PTH levels are therefore common in kidney failure but aggravate
renal bone disease.
How is renal bone disease monitored?
The best guide to progress and severity of renal bone disease is
the amount of PTH in the blood, which is obtained by a simple
blood test. Changes in blood PTH can tell us about what will
happen to the bones in the future; the lower the PTH, the
As renal bone disease can be present before dialysis is needed,
doctors will often measure a patient's blood PTH before dialysis
is required. Once a patient is having dialysis, it is usually
measured every six months. If the patient has a high blood PTH,
treatment is commenced to prevent the problem worsening. High
PTH levels can usually be lowered with medication, but
occasionally removal of the overactive parathyroid glands is
How is renal bone disease treated?
Treatment may be needed for each of the four main causes of
renal bone disease.
1. Treatment of low
vitamin D levels - Vitamin D treatment is also needed to
prevent renal bone disease. The most common form of treatment is
with alfacalcidol, which provides the active vitamin D that is
lacking and therefore increases blood calcium levels. Peritoneal
dialysis patients receive vitamin D in the form of a tablet, and
haemodialysis patients receive it either as tablet, or as an
injection during dialysis.
2. Treatment of high
phosphate levels - Dialysis removes some phosphate from
the blood but most renal patients need further treatment to
control phosphate levels Phosphate binders such as calcium
carbonate need to be taken just before food to be effective. The
target blood phosphate level is less than 1.8mmol/l for a
patient on dialysis.
3. Treatment of low
calcium levels - Patients on dialysis obtain some extra
calcium from the dialysis fluid whether on PD or haemodialysis.
This happens because there is more calcium in the dialysis fluid
than in the blood. Calcium passes from the stronger solution
(the dialysis fluid) into the patient's blood (the weaker
solution) by a process called diffusion. Sometimes extra calcium
from the fluid is not enough. Therefore they are given extra
calcium in form of a drug called calcium carbonate. Although
calcium carbonate is used to reduce blood phosphate levels, it
can also raise blood calcium levels. Blood calcium levels are
also raised by treatment with vitamin D.
- In most patients, correcting the blood levels of calcium,
phosphate and vitamin D is enough to control renal bone disease,
and to cause PTH levels to fall. In some patients, the blood PTH
levels continue to rise. When this happens the blood calcium
tends to rise to above normal (it is usually low in kidney
failure), and the blood phosphate is very high. This combination
of an extremely high PTH, a high calcium and a very high
phosphate level cannot be treated by dialysis and tablets alone.
It is then necessary to carry out an operation to remove the
parathyroid glands. This operation is called a parathyroidectomy.
If the operation is not performed, the blood vessels can become
"furred up" with calcium, the eyes can become red and itchy and
the skin can also become affected due to the calcium deposits. A
parathyroidectomy is a very effective operation. It returns
blood calcium levels to normal and can prevent these
complications worsening. After the operation, frequent checks of
the blood calcium level are needed as they can fall to a very
low level after the operation. It is often necessary for
patients to take high doses of calcium carbonate/ or vitamin D
after a parathyroidectomy. These can usually be stopped at a