Each month people on dialysis meet with their renal dietitian to go over the lab work from their monthly blood test. Calcium, parathyroid hormone (PTH) and phosphorus are among the lab results that are discussed. The dialysis health care team monitors these because it is how they determine bone health and provide mineral and bone disorder management.
Many people are aware that healthy kidneys clean waste from the blood and produce urine; however, kidneys are also responsible for keeping minerals in the body in balance, including the ones that help keep bones healthy.
Having healthy bones is essential to maintaining body structure and mobility. The human skeleton supports body weight and protects the brain and other organs. The skeleton also stores two important minerals — calcium and phosphorus.
Bone is created by bone cells. The two main types of bone cells are osteoblasts and osteoclasts. Osteoblasts are cells that build bone. Osteoclasts are cells that consume bone. These two types of cells are in balance in a healthy body, and bones are being built up and torn down at all times. In people with bone disease related to kidney failure, osteoclasts and osteoblasts are often not in balance. This condition is the bone disease called renal osteodystrophy. The way these bone cells get out of balance is when calcium, parathyroid hormone (PTH), phosphorus and activated vitamin D are out of balance.
Over time, renal osteodystrophy can lead to bones that become brittle and chalklike, causing them to break easily, hardening of the soft tissues of the body including the heart and may even lead to a higher death rate in people with end stage renal disease (ESRD).
Calcium is the most plentiful mineral in the body that is essential for building new bone cells and keeping existing bones strong. Many foods such as milk, yogurt, cheese and canned fish with bones are rich with calcium, but also provide too much phosphorus for a kidney diet. Sometimes calcium pills are prescribed to provide extra calcium instead of these high-phosphorus foods for people with chronic kidney disease (CKD). If calcium levels in your blood become too low due to kidney failure, your parathyroid glands (four small glands in your neck) release a hormone called parathyroid hormone (PTH). This hormone regulates calcium levels in your blood. If blood calcium levels are too low, the PTH hormone will begin removing calcium from your bones to get calcium blood levels back to normal. Over months and years, as calcium is stripped from the bones, this can make the bones weak, so their texture becomes more like a piece of chalk than of a sturdy bone.
After calcium, phosphorus is the second most common mineral in the body. About 85 percent of phosphorus in your body is stored in your bones and teeth. Phosphorus is found in many foods — milk and milk products, whole grains, dried beans and peas, nuts and seeds, organ meats, meat and fish, colas, chocolate and some types of baking powder. In addition, many foods contain phosphate additives added during food processing. Your renal dietitian will generally recommend that you limit or avoid foods high in phosphorus, as part of mineral and bone disorder management. Many people with kidney disease and those on dialysis are prescribed phosphorus binders to take when they eat to prevent the body from absorbing phosphorus from the foods they eat.
Extra phosphorus is removed from the body by healthy kidneys, but when your kidneys don’t work, phosphorus builds up in your blood. A high blood phosphorus level causes your body to pull calcium from your bones in an attempt to balance all the body’s minerals. When calcium is being pulled from your bones, your bones begin to break down and lose the ability to provide structural support. Excess phosphorus also causes hard deposits in the body’s soft tissues — blood vessels, heart, lungs, skin and eyes. High phosphorus levels may also be the cause of severe itching of the skin for people with end stage renal disease.
Healthy kidneys activate vitamin D. The activated form of vitamin D is called calcitriol. Calcitriol helps the body absorb calcium. Working together, calcitriol helps maintain normal parathyroid hormone (PTH) levels, and they carefully balance calcium in your system. When kidneys fail, they stop converting inactive vitamin D to calcitriol. The result is your body is unable to absorb calcium from food, so it “borrows” the calcium it needs from the greatest calcium storage depot — your bones.
Kidney disease patients whose kidneys aren’t making enough calcitriol are prescribed a special active vitamin D supplement, and your doctor may also prescribe a calcium supplement if needed. On the other hand, if your calcium level is too high, your doctor may prescribe a non-calcium-based phosphorus binder and withhold vitamin D therapy until calcium levels are normal.
People in later stages of kidney disease are at risk for renal osteodystrophy. Renal osteodystrophy is often called the “silent crippler” since kidney patients may not experience symptoms at all. Usually symptoms do not occur until a patient has been on dialysis for a few years. Typical symptoms can be:
Early indicators of renal osteodystrophy include high phosphorus and/or high parathyroid hormone (PTH) levels, red eyes, itching and sores from calcium-phosphorus deposits.
Children with kidney disease can be especially affected by renal osteodystrophy because their bones are still growing. Even before children with kidney disease begin dialysis, they can have symptoms of renal osteodystrophy. This bone disease can lead to slow bone growth in children and can cause “renal rickets” where the leg bones grow either bowed toward each other or away from each other. Children with renal osteodystrophy are also prone to be shorter in stature.
Because there are different types of bone disease, your doctor will test you specifically for renal osteodystrophy if you are experiencing symptoms such as bone pain or deposits of calcium under your skin or if blood test suggests that you suffer from renal osteodystrophy. Testing for renal osteodystrophy involves taking a blood sample to measure your levels of calcium, phosphorus and parathyroid hormone (PTH). If you are on dialysis, calcium and phosphorus tests are done on a monthly basis (or sometimes more frequently). PTH is measured quarterly for most patients — although testing may be done more frequently for people just starting vitamin D therapy or those with severe bone disease while the health care team is determining the correct dosage of vitamin D. Your renal dietitian will review your lab results with you and recommend treatments such as changes in your diet or change your phosphorus binder prescription. Depending on your situation and risk factors, your doctor may also take a bone biopsy from your hip to see how dense your bones are. After reviewing the results of your tests, your doctor will recommend an appropriate treatment regimen if renal osteodystrophy is found.
The goal of treating renal osteodystrophy is to restore balance between calcium, parathyroid hormone (PTH), phosphorus and vitamin D in the body. Renal osteodystrophy can be managed with phosphorus binders, activated vitamin D and a low-phosphorus diet. If you have a high level of PTH in your blood, it’s important to bring it back to a normal level to prevent calcium loss from your bones. Medicines to treat renal osteodystrophy can be given in pill form, but are usually given intravenously during dialysis treatments for people on in-center hemodialysis. These medicines are only used in patients with kidney disease, since normal kidneys are able to activate the vitamin D that is normally made by the body. Another drug that may be used acts directly on the parathyroid glands to block PTH release. In severe cases the parathyroid glands may be surgically removed.
In addition to a low-phosphorus diet and taking prescribed medicines, exercise can also help increase your bone strength. Check with your doctor before beginning an exercise program.
As you can imagine, mineral and bone disease management can be complicated. While this article gives you an overview of the minerals and hormones involved in renal osteodystrophy, your health care team has a vast knowledge of how your body works. If you have kidney disease — and especially if you are on dialysis — talk to your doctor or renal dietitian about renal osteodystrophy and your mineral and bone disease management. Through following the kidney diet, getting adequate dialysis and taking various medicines, you can help control bone disease and work to keep your bones healthy and strong.
This site is for informational purposes only and is not intended to be a substitute for medical advice from a physician.
Please check with a physician if you need a diagnosis and/or for treatments as well as information regarding your specific condition. If you are experiencing urgent medical conditions, call 9-1-1