I wrote this article a number of years ago. You might want to explore the links below instead of this page to read more current information about kidney disease in your pet
A Complete 2018 Update On Kidney Disease In Dogs & Cats Go Here
hyperthyroidism a common cause of kidney disease in cats
Much of this article has become a stub to the newer and more detailed links above. All
of our bodies suffer the wear and tear of time. In people, our hearts
are often our weakest organ. In dogs and cats, it is often the kidneys
that wear out first.
There is a slow, but steady, loss of kidney function in all of our pets as they age - so much so, that next to arthritis, chronic renal (kidney) failure is the leading cause of illness in older dogs and cats.
First, you need to know something about the work of the kidneys. Kidneys keep your pet’s body free of the wastes that accumulate during metabolism. They are continually scrubbing the blood free of excess salts, water and metabolites.
The actual removal of wastes occurs in tiny systems within the kidneys called nephrons. There are almost one million of these structures in a single human kidney, no one has counted them in dogs or cats. Each nephron contains a small sieve-like filtering structure called a glomerulus. These glomeruli (plural form) keep normal blood proteins and cells in the bloodstream, while allowing extra fluid and wastes to pass through to end up in the pet’s urine. A complicated chemical exchange takes place, as waste materials and water leave the blood and enter the urinary system. The kidneys also regulate the body’s acidity and, through regulation of body salt content, they help control blood pressure. Cells associated with healthy nephrons produce an important hormone called erythropoietin and and enzyme called renin. Erythropoietin is necessary for the pet’s body to produce and maintain red blood cells while renin activates another hormone (angiotensin) to helps control blood pressure. In addition, healthy kidneys are required to process vitamin D into calcitriol to preserve calcium for bones and for normal calcium balance in the body In chronic kidney disease these glomeruli are scarred and lost, or plugged up with proteins and inflammatory cells. Without enough functioning glomeruli, none of the processes I have mentioned work normally.
The animal body is marvelous in sensing when it has a problem. In an attempt to keep the body waste-free, your pet’s kidneys work overtime, using their small remaining capacity to remove waste. This accounts for the excess thirst and urination you have seen in your pet. For a while, this compensation keeps it’s body clean enough of wastes to function, but gradually, the pet can not consume enough water to keep waste levels in check. By the time your pet experiences weight loss, anemia, and abnormal blood work results, over half of its kidney glomeruli have been lost. You pet can not replace them.
The first sign that there is a problem is when your pet begins to drink water and urinate excessively. At first, it is normal for owners to ignore this. It might just be that your dog wakes you up during the night to be let out or that your cat’s water bowl had to be filled more than it used to.But with time, the pet begin to loose weight and become a more finicky eater. About this time, the pet’s energy levels tend to decrease. They play less, romp less and sleep more. Generally, their coat lacks the luster it once had. This is often when pets are first taken by their concerned owner to see their veterinarian. In advanced kidney disease, pets will no longer eat. They often have digestive disturbances such as nausea, retching and diarrhea. Their water intake decreases and they become dehydrated. They may stand over their water or food bowl without attempting to eat or drink. These pets have developed uremia – an intolerably high level of nitrogen-containing metabolic waste products in their blood. Because these toxic waste products all contain the azo-molecular grouping of nitrogen, another term for uremia is azotemia.
Veterinarians know the things that make a pet’s kidneys fail suddenly. We are much less certain why they are more likely to fail gradually. Many causes have been discussed that seem logical – but few of them have been proven to be true. Regardless of the cause, all cases of chronic kidney disease develop the same signs and pass through the same stages. Usually, your veterinarian will just tell you your pet has CRF. This is because, in most cases, there is no way for the veterinarian to determine the cause.
There was a time – not so long ago – when infectious diseases and dietary deficiencies ended the lives of dogs and cats early. But with advances in pet nutrition, antibiotics and sophisticated surgery, our pets now live much longer. Nothing lasts forever and every organism has its weakest link. Cells of the kidney cannot replace or regenerate themselves as they do in the liver, lungs, bone and skin. Once a glomerulus ages and is lost, it is lost forever. This is probably the most common cause of kidney failure in dogs and cats.
Some cats and dogs were destined from birth to loose kidney function too early in life. These pets inherited genes that cause fluid-filled sacks (cysts) to form within their kidneys. As these cysts gradually grow in size, they crowd out and destroy the functional tissue (glomeruli) within the pet’s kidneys. This is an inherited problem in certain purebred cats. It is much less common in dogs, but it does occasionally occur in them – particularly in terriers and beagles.
Chronic interstitial nephritis is the most common form of kidney damage in older dogs. It occurs less frequently in cats. Nephritis is a term for inflammation of the kidneys. The tissue that surrounds the nephron filters is called the interstitial tissue. It is the matrix that suspends the nephrons - much like stars are suspended in space. Pathologists that examine kidney tissue from pets with failing kidneys have noticed that many have a higher than normal number of inflammatory cells invading this area. This low-grade, chronic inflammation is thought to cause scaring that eventually destroys most of the nephron filters. Acute (sudden) nephritis can occur in dogs that are infected with leptospirosis. Leptospirosis is diagnosed less frequently in cats. After the acute phase of this disease, the organism responsible sometimes lingers for long periods of time in the pet’s kidneys, causing a chronic nephritis. However, most dogs with chronic interstitial nephritis show no evidence that they were ever infected with leptospirosis. Pets suffering from chronic interstitial nephritis have small, shrunken, hard kidneys due to scarring. If the pet is not too chubby, it is often easy to palpate and identify these shrunken firm kidneys during a routine veterinary exam.
Your pet’s kidney glomeruli act as a sieve, straining and filtering blood as it passes through it. Very large molecules in the blood have a tendency to collect there and appear to slowly damage the kidney’s filtering-ability. Some of these large molecules are antibodies combined with antigens (immune complexes). Many chronic infectious and auto-immune diseases produce immune complexes. These include Lyme disease, chronic skin infections, chronic intestinal disease, overactive adrenal glands (Cushing's) and diabetes. Chronic gum disease (periodontal disease) is associated with kidney damage in humans. It is associated with heart disease in dogs. We do not yet know if it is a risk factor for kidney disease in pets. A type of destructive protein sometimes accumulates in the kidneys. It is called amyloid (amyloidosis). In some cats, this is a genetic disease. But it is also know to occur subsequent to long-term over-stimulation of the immune system. Abyssinians and Siamese cats, shar pei and akita dogs all have a higher than normal incidence of amyloidosis which can lead to kidney failure. A similar form of kidney damage in pets occurs in auto-immune diseases that are similar to lupus in humans. In this disease, run-away antibodies are produced against the pets own body. In some cases, these antibodies are directed at the pet’s kidneys themselves, in others, they may only accumulate there causing physical damage.
An overactive thyroid gland or hyperthyroidism has become a very common problem in older cats. You can read an article on this problem here to see what some of your options are. We are uncertain why it is occurring more frequently, but we know that it often occurs concurrently with kidney disease. Hyperthyroidism often masks the signs of kidney failure, and it is only when your veterinarian resolves your cat’s thyroid problem that it becomes apparent that the cat’s kidneys are failing. We know that hyperthyroidism may cause your cat’s blood pressure to be abnormally high. We also know that high blood pressure leads to kidney failure. This is probably why hyperthyroidism and kidney failure go hand-in-hand in cats.
Feline Urological Syndrome (FUS) is another disease that has become very common. In this condition, a pasty grit (struvite crystals) irritates and sometimes plugs the cat’s urethra, preventing normal urination. When the urethra is partially or completely plugged and the cat cannot pee, urine pressure builds up in the bladder, up the tubes to the kidneys (ureters), and into the kidneys themselves. Abnormally high urine pressure in the kidneys slowly destroys them. The condition is called hydronephrosis. However most cats that loose their normal kidney function do not show the kidney changes associated with hydronephrosis. In dealing with FUS, cats are often placed on diets that are very acidic, in an attempt to prevent struvite crystals from forming. Some veterinarians believe that the acid urine these diets produce is unhealthy for the kidneys and may be one reason that they fail. The pH of these diets has recently been adjusted upward to take this into account.
The history you give your veterinarian, your pet’s age and the veterinarian’s physical examination of your pet may make your veterinarian suspect a chronic kidney problem. As kidneys scar, they become hardened, small and lumpy. In lean pets, they have a characteristic feel when felt through the abdominal wall. In these cases, and when a diagnosis is unclear, your veterinarian will run tests. Blood and urine tests that warn of kidney damage are included in all standard laboratory examinations. When your pet feels poorly and the cause is uncertain, these are the first tests your veterinarian will run (for normal results in dogs and cats, see my article on normal blood values).
When your veterinarian asks you to bring in a urine specimen from your pet, its specific gravity will be checked. This tells your veterinarian how concentrated the urine sample is. Pets that have weakened kidneys cannot produce concentrated urine. The lower the specific gravity, the more serious the kidney problem is likely to be. However, anything that causes your pet to drink excessively will also lower urine specific gravity. That is why it is wise to collect your pet’s urine specimen as soon as possible after it rises in the morning and before it has consumed water. For certain analysis, it is better if the veterinarian collects the sample.
Failing kidneys leak blood proteins into the urine. Most of this protein is albumin. A high urine protein content is often an early sign of sudden or long-term kidney damage. The presence of white blood cells and debris in the urine help veterinarians tell the difference between sudden (acute) and chronic (long-term) kidney disease.
Blood urea nitrogen, a waste product of metabolism, rises in the blood of pets with failing kidneys. It’s level stays within a relatively narrow range in the blood of healthy pets. BUN level in the blood of ill pets begins to rise when not enough healthy kidney tissue remains to excrete it into the pet’s urine. The higher its level in the blood - the more serious the kidney problem is likely to be. Blood urea nitrogen and creatinine levels are the prime way veterinarians decide if their treatment of kidney disease in your pet is working. You will have many of these tests run on your pet during therapy.
Creatinine is a protein metabolite of muscle that tends to rise and fall in tandem with BUN. Creatinine-determination is a more sensitive test for kidney disease then BUN-determination because blood levels of creatinine fluctuate less than urea nitrogen in response to a pet’s being dehydrated or consuming a high-protein meal. So BUN and Creatinine tests are almost always run together. The results are often expressed as a BUN:Creatinine ratio.
Phosphorus is one of the mineral constituents of blood. The foods your pet consumes are very high in phosphorus. It's failing kidneys have difficulty excreting sufficient phosphorus into the urine. An elevated blood phosphorus level is another sign of failing kidneys. As the ratio of phosphorus to calcium in the blood becomes abnormal, the pet’s bones will weaken. This is why pets in kidney failure need to be fed diets low in phosphorus. Pets with kidney damage may also loose their ability to produce calcitriol. When this occurs, they can no longer absorb sufficient calcium from the foods they eat.
Proper internal levels of potassium are very important to your pet's well being. When a pet’s kidneys fail, its body potassium levels rise. This problem, called hyperkalemia causes generalized fatigue, nausea and an irregular, slow heartbeat that can be life threatening. However, when pets with advanced kidney disease loose their appetites, their blood potassium level can fall dangerously low.
Your pet’s packed cell volume is a measure of possible anemia. When a pet with kidney failure has a PCV that is abnormally low, it is not manufacturing sufficient red blood cells. One of the hormones involved in red blood cell manufacture is produced in the kidneys. It is called erythropoetin. When your pet’s kidneys deteriorate, not enough of this hormone is produced.
You veterinarian may also measure your pet’s blood pressure. It is common for pets with chronic kidney disease to also have abnormally high blood pressure. It is unclear if the high blood pressure is part of the cause of kidney damage, or the result of kidney damage. High blood pressure is known to damage the kidneys – but kidney disease is also known to elevate blood pressure. This is called secondary hypertension.
In the future, we may be able to regenerate failing organs. But for now, there is no known way to mend damaged kidneys. What veterinarians can do is to try to slow the rate at which your pet's kidney tissue is lost and deal with the side effects of the loss. Kidney failure is progressive – that means that with time it will get worse. The key to gaining time for your pet is to use the its remaining kidney tissue as efficiently as possible. We try to do this through diet, medications and, when necessary, fluid injections (diuresis).
Commercial diets, designed for kidney failure are considerably lower in protein (1/3 - 1/2 the amount) and sodium than ordinary pet foods. They also have added omega 3 and omega 6 fatty acids and compounds like potassium citrate to counteract body "over-acidity" and they are drastically lower in phosphorus. But your pet's health on protein restricted diets needs to be monitored carefully. Blood tests need to be done periodically to be sure that its blood protein levels have not dropped too low and that the pet's body weight remains stable. When you do that, and the pets BUN and Creatinine levels drop or remain stable, protein restriction is a very positive step. But there are periods in a pet's ongoing fight with kidney disease when restricting protein might not be a good thing to do. (For example, when 7/8th of its kidney's filtering apparatus has been lost) (ref1 ) (ref2) Cats do not tolerate low protein diets as well as dogs. Cats do not metabolize added carbohydrates as well. (ref) It may be wiser to depend more on added fat and fiber for dilution of the cat's protein consumption rather than a large amount of added plant carbohydrates. (Higher fat diets can be beneficial to kidneys. (ref)) Ketoacids, as sold through body building outlets, can also act as a substitute for dietary protein in certain instances (ref) (But I know of no veterinarians that use them in dogs and cats).
No matter what you decide to feed, we always want to limit your pet's consumption of phosphorus. The foods naturally highest in phosphorus are the common high-protein foods, meat, poultry, fish, dairy products, peas and beans. Limiting the amount of sodium your pets ingests is also wise when its kidneys are failing - so commercial-prepared kidney diets limit the amount of sodium-rich ingredients in their foods. They also add omega-3 and omega-6 fatty acids that are found in cold-water fish and fish oils combined with flax seed. In advanced kidney disease, when your pet's BUN is over 60 mg/dl, most vets believe that moderately restricting protein in your pet's diet does become important.
Pets with kidney problems often have poor appetites, weight loss and anemia. They may suffer digestive disturbances as well that could limit the absorption of vitamins. So B-vitamins are often given as appetite stimulant and to ward off any deficiency.
An organ as basic as the kidney does not appear to vary much between mammals. Experiments in kidneys disease are most acceptable to animal welfare advocates these days when they are done in rats. The cells that researches zero in on in declining kidney function are the podocytes , cells in the kidney's filtering apparatus that underlies its blood-cleansing abilities. Once podocytes loss begins, like a tree, bent to a severe angle by a storm - it will continue to slowly fall even after the wind ceases even though the remaining filers "super nephrons" enlarge (hypertrophy) and work harder. There is considerable evidence that medications called ACE inhibitors can slow that loss. (ref) In fact, ACE inhibitors might actually restore or aid in renal (kidney) repair. (ref) The ones most often chosen in pets are benazepril. and enalapril If your pet is placed on an ACE inhibitor, it is wise to be sure that its blood creatinine levels do not increase. In later kidney failure when the remaining kidney filters (glomeruli) are filtering way above their normal capacity, ACE inhibitors occasionally drop the kidney's internal pressure so low that the pet's uremia actually worsens. The best monitoring test in those situations is a 24 hours creatinine clearance test or another test that estimates the pet's GFR.
Certain compounds called phosphate binders can block the absorption of phosphorus from your cat's foods while it is still within its digestive system. At one time, aluminum hydroxide was suggested. Dieticians now think that more modern products that are free of aluminum are safer. Some common ones are calcium acetate (PhosLo), sevelamer (Renagel) and Epakitin®. There are others in development. (ref)
Since pets with advanced kidney disease may not produce adequate amounts of active vitamin D in their kidneys, the preformed compound, calcitriol, can be given to them. It is generally given when blood calcium/phosphorus levels and ratios become abnormal.
Potassium supplements (Tumil K, etc.) help when the pet’s blood potassium level drops too low. This sometimes helps combat the listlessness and weakness that accompanies advanced kidney failure.
Sold as Epogen, Betapoietin or Eporel, these compounds encourage your pet to produce red blood cells and so combat anemia. Because these compounds were engineered for humans , dogs and cats eventually cease responding to them. But they often do raise the pets PVC for a time. There is a danger in giving this product. When the pet's immune system decides to attack human erythropoetin as a foreign protein, it not only destroys the human erythropoetin that was given - it also destroys the pets natural erythropoetin. So it can make the anemia even worse. It should only be used as a last ditch effort.
Fermentable or soluble fiber, when added to a pets diet, also helps remove toxins from its body. Because of this, it is often an ingredient in commercial diets sold to manage kidney failure in pets. In these diets, the source is sugar beet pulp. It is sold in quantity to stables as a horse feed additive.
There comes a time with all pets when they no longer drink enough water on their own to fully utilize their remaining kidney capacity. Early in this period, you can give your pet additional fluids orally or add additional liquid to its food. When that is no longer sufficient, the fluids needs to be give periodically under the pet’s skin by injection. The effect is called diuresis. Its effect in flushing out lowering blood toxins from your pet can be dramatic. Many owners learn how to administer these subcutaneous fluids at home. In most cases, there is no benefit in giving them intravenously. Pets with failing kidneys do need emergency intravenous fluids when they are presented severely dehydrated to veterinarians.
That is completely dependent on the level of toxins in your pet’s blood. Pets with blood creatinine levels below 2.8 mg/dl usually do well for long periods. Pets with blood creatinine levels of up to about 4 mg/dl have also survived happily for many years with appropriate treatment. But when your pet's creatinine levels exceed 5, its quality of life has become quite poor. Level of 5 and above mean that 80-90% of their kidneys have been destroyed. It is possible to keep these pets alive – but I question the kindness of doing this. Your pet loves you very much. But it is a two way street - it is relying on you to end its life peacefully and humanely when the right time comes.
Kidney transplants are an option for pets if you are fortunate enough to be able to afford them. They are much more successful in cats than in dogs. Currently, a little more than half of the cats that have kidney transplants survive six months. Of those that do, many have lived an additional three years. Success rates for transplant surgery generally go up as specific veterinary centers gain more and more experience with procedures. Transplantation surgery in cats is still in its infancy. Success rates vary from one veterinary center to another. It is not just the expertise of the surgeons that accounts for this. Some Centers are willing to try transplant surgery on pets that are already seriously ill. In those cases, the overall success rate will be lower than at Centers that confine their surgery to more healthy pets. Dogs do not fare as well with kidney transplants. The biggest obstacle to kidney transplantation in dogs is rejection of the new kidney. Powerful immunosuppressive drugs must be given to the dog for the rest of its life. These drugs have serious side effects of their own. Centers that once performed the procedure on dogs have ceased to do so. But others are always begin programs that attempt to get around the hurdle of rejection in novel ways.
Hemodialysis, as performed on humans with failing kidneys, is not done frequently in pets. In September of 2008, the veterinary school, University Of Florida released as press report of a successful hemodialysis procedure on a dog. I do not know if other Centers are attempting it.
Undoubtedly, some of the medications that are currently being tried by veterinarians on cats and dogs with kidney failure will prove to be beneficial. The only way we make progress in medicine is through experimentation. However, as in the case with every disease of humans and animals, the majority of these new medications will be found to not be helpful. If traditional medications and procedures are longer helping your pet, there is no harm in trying an experimental therapy. Things that are sold over the Internet with marvelous claims to desperate owners are always worthless.