Bones, Calcium, Phosphate, and PTH in Kidney Failure
The problems discussed in this section usually only develop in people who have had kidney function down to less than 40% of normal. More rarely, calcium and phosphate problems can occur in people with other kidney diseases. If you are not someone with kidney failure, on dialysis, or with a kidney transplant and you are aware of some calcium or phosphate problems, you should discuss these with your doctor on an individual basis.
Bone disease is very important for people with kidney failure. Once serious problems have developed, they cannot be fully reversed. Therefore prevention is better than cure’. Blood tests can identify problems at an early stage, and treatment can be altered to reduce the chances of serious problems developing.
Unfortunately, this is a complicated area. Although the authors have tried to explain everything as simply as possible, it is still not easy. The good news is that it is not necessary for people with kidney disease to understand all the details, so don’t worry! Look at the key points immediately below and then any particular issues that affect yourself, and the rest is only there for your information.
Key action points
- Understand that some problems with calcium and phosphate can make you feel ill right away, but other problems with your bones or your heart can develop slowly over years, so that preventative treatment will help keep you well.
- Discuss targets for treatment with your Renal Unit – the simplest target is to keep the blood phosphate level below a certain level (ideally 1.8 mmol/l, though 2.0 mmol/l is a more realistic target in some people).
- Understand your diet, so that you have the correct daily intake of phosphate.
- Take medication to reduce the phosphate levels regularly as prescribed, and discuss honestly with your Renal Unit if you are unable to take the medication as prescribed.
This is a complicated topic. Is it important?
Problems with calcium, phosphate and a chemical messenger in the blood called parathyroid hormone (PTH for short) can occur in anyone with kidney failure. The level of kidney function at which problems start to become apparent is about 40% of normal, or an eGFR of 40 ml/min. Sometimes there are symptoms – in other words someone feels unwell. In many cases, however, problems can start without any symptoms, and slowly cause irreversible damage to the bones, the heart or blood vessels over a period of years.
Preventative treatment can reduce the chances of this irreversible damage occurring, but requires a lot of care and energy from the patient (yourself) and the renal team in hospital.
What is calcium?
Calcium is a mineral that is found throughout the body. It makes up, together with phosphate, the main strength in the bones. Calcium is also used to help ‘power’ muscles, and is carried around the body in the blood. The blood calcium level must be kept very tightly controlled for the body to work normally, and PTH (see below) is important in this. Calcium gets into the body from food, and is found particularly in dairy products, green vegetables and eggs.
The normal calcium level in the blood is between 2.2 and 2.6 mmol/l (millimoles per litre).
What is phosphate?
Phosphate is a mineral in the body, and together with calcium makes up most of our bones. Phosphate, like calcium, is also used in other parts of the body to ‘power’ muscle and is used in many other chemical reactions. Phosphate gets into the body in food. Dairy products, nuts and meat are three types of food that can contain a lot of phosphate.
The normal level of phosphate in the blood is 0.8 to 1.4 mmol/l.
What is PTH and why is it important?
Parathyroid hormone (PTH for short) is a hormone (chemical messenger) which is very important in controlling the level of calcium in the blood. PTH is produced in the parathyroid glands. There are usually four parathyroid glands, each the size of a pinhead (0.1cm). They are found in the neck, behind the thyroid gland. The thyroid gland has completely different functions from the parathyroids, they just happen to be next to each other.
If the level of calcium in the blood falls, the parathyroid glands normally produce more PTH, which pulls some calcium from the bones into the blood, normalising the level. If the calcium level in the blood rises above normal, PTH secretion falls, and the level of calcium in the blood falls back to normal.
The normal range for PTH depends on the technique used by an individual laboratory, please check with your Renal Unit if you want to know your local normal range.
What is Vitamin D and why is it Important?
Vitamin D is a chemical needed so that calcium can get from food into the body. A little Vitamin D is absorbed from food, but most is made in the skin, in a process that only occurs if the skin is stimulated by sunlight. Even then, vitamin D has to be converted to an active form in the kidneys.
Vitamin D blood levels are not routinely measured by most Renal Units..
What can go wrong with Calcium, Phosphate, PTH and Vitamin D in Kidney Failure?
If someone developed kidney failure and went onto dialysis, the following sequence of events might occur. However, preventative treatment can stop many of these problems.
- 1. A fall in the blood level of calcium is the first major change. As the kidneys do not convert vitamin D into its active form, calcium does not get into the body from food, and the blood level of calcium can fall. The treatment is to replace active vitamin D (more details above).
- 2. Levels of phosphate in the blood rise, because the kidneys are not excreting excess phosphate into the urine. High levels of phosphate can cause itching. The treatment is to reduce phosphate levels by diet, dialysis, and medication (more details below).
- 3. PTH may be produced in large quantities, stimulated by low levels of calcium in the blood. The PTH tries to keep the calcium level in the blood normal by increasing calcium absorption from food, but also takes calcium out of the bones. Eventually the parathyroid glands work so hard they go out of control, and cause persistently high calcium levels. This may require treatment to reduce the PTH levels which may be through a tablet (Cinacalcet) or an operation to remove the glands (the operation is called a parathyroidectomy, more details below).
- Low calcium
- High calcium
- Low phosphate
- High phosphate
- Low PTH
- High PTH
There are several types of treatment used. No one treatment is completely effective, largely because dialysis cannot replace 100% of kidney function, so there is almost always a tendency to high phosphate levels.
- Good quality dialysis.
- Vitamin D medication.
- Medication to reduce phosphate absorption (using phosphate binders).
- Kidney transplantation. Unfortunately this does not put everything back to normal
- Medication to lower blood calcium levels (calcimimetics)
In some people the parathyroid glands go out of control and produce far too much PTH. In some cases the only treatment is surgery on the neck to remove the parathyroid glands.
What is Calciphylaxis?
This is a medical term for the deposition of calcium and phosphate in parts of the body other than the bones. There can be a build up of ‘chalk’ (chalk is calcium and phosphate) in blood vessels, heart valves or other parts of the body. One of the reasons for controlling phosphate levels is to avoid calciphylaxis. Calcium deposition in the blood vessels may occur rapidly in people on haemodialysis who have high phosphate levels. Although the calcium is deposited in a manner that is different from the narrowing of the arteries that cause heart attacks, there can be other serious problems with the arteries. One of these is to make it impossible to have a kidney transplant. Sometimes the surgeons only discover how calcified blood vessels are at the time of surgery, and they have to abandon the operation. This makes good phosphate control even more important.
What causes Pain in Bones and Joints?
Pain in bones or joints is a common problem in people with kidney failure, or with kidney transplants. It may be due to muscular sprains or arthritis which, of course, may occur whether there is kidney failure or not. Some types of bone disease may not cause pain, others can be painful. It is necessary to talk to your medical team to make a diagnosis – this webpage is only designed to give helpful information after a diagnosis has been made.
There are some simple treatments you can use for bone and joint pains, once a more serious cause needing specific treatment has been ruled out (click here for more details on these treatments).
Types of bone disease that can occur in people with Kidney Diseases
Many people with kidney disease do not develop bone disease. However, if they do, there are several types that may occur. Some of these are related to the calcium and phosphate problems mentioned above, and others are not. Some types of bone or joint disease may have developed even if the person had never had any problem with their kidneys.
Some diseases frequently seen in people with kidney diseases are listed here. Other conditions seen less often (such as rheumatoid arthritis) are not included ...Click here to go the Arthritis Research Campaign (UK) website which contains details on various types of arthritis.
- Bone disease due to high PTH (called renal osteodystrophy)
- Adynamic bone disease
The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.