PU/PD stands for polyuria/polydipsia, the medical shorthand veterinarians use when a dog is urinating more than normal and drinking more water to match. It’s not a disease itself but a clinical sign that something is driving the body to process far more water than usual. Normal water intake for dogs falls between 20 and 70 ml per kilogram of body weight per day, and intake above 100 ml/kg/day is considered definitively excessive.
PU/PD is one of the most common reasons dog owners bring up concerns at vet visits, and it can point to a wide range of underlying causes, from highly treatable conditions to serious chronic disease.
How PU/PD Works in the Body
There are two directions PU/PD can flow, and understanding the difference helps make sense of the diagnosis your vet is chasing. In most cases, the dog is losing too much water through urine first, then drinking more to compensate. This is called primary polyuria with compensatory polydipsia. Something is wrong with the kidneys, the hormones that regulate urine concentration, or the bloodstream chemistry, and the body can’t hold onto water the way it should. The dog gets thirsty because it’s becoming dehydrated from all that urine output.
Less commonly, the problem starts with drinking. In primary polydipsia, a dog drinks excessively for behavioral or neurological reasons, and the kidneys respond by flushing out the extra fluid. The kidneys are working correctly here. They’re just dealing with a flood of incoming water by reducing the hormone that tells them to hold onto it. Over time, this creates a cycle of heavy drinking and heavy urination that can look identical to the first pattern from the outside.
The Most Common Causes
The three conditions veterinarians see most often behind PU/PD are kidney failure, Cushing’s disease (hyperadrenocorticism), and diabetes mellitus. These account for the majority of cases, and they’re typically what your vet will investigate first.
Kidney Disease
The kidneys concentrate urine by filtering blood through millions of tiny structures called nephrons. As nephrons are damaged or lost, the kidneys gradually lose that concentrating ability. PU/PD is usually the earliest noticeable sign of kidney disease, but it doesn’t appear until roughly two-thirds of the nephrons are already gone. That’s why a dog can seem perfectly healthy for a long time before owners notice the water bowl emptying faster or more frequent trips outside. By the time drinking and urination increase, the kidneys are already working at a fraction of their full capacity, though the dog may still be in an early, manageable stage of chronic kidney disease.
Cushing’s Disease
Cushing’s disease causes the body to produce too much cortisol, a stress hormone. Excess cortisol interferes with the hormone that normally tells the kidneys to reabsorb water and concentrate urine. The result is a steady stream of dilute urine and a dog that seems to always be at the water bowl. Cushing’s is especially common in middle-aged and older dogs and often comes with other signs like a pot-bellied appearance, hair thinning, and increased appetite.
Diabetes Mellitus
When blood sugar rises too high, the excess glucose spills into the urine and pulls water along with it through a process called osmotic diuresis. This creates large volumes of urine regardless of how much the dog drinks. Dogs with diabetes often also lose weight despite eating well, and they may seem lethargic.
Other Conditions That Cause PU/PD
Beyond the big three, a long list of conditions can trigger increased drinking and urination. High blood calcium levels (hypercalcemia) interfere with the kidneys’ concentrating ability and can stem from certain cancers, overactive parathyroid glands, or other metabolic problems. Liver failure and portosystemic shunts, where blood bypasses the liver through abnormal vessels, also commonly cause PU/PD. Pyometra, a serious uterine infection in unspayed female dogs, is another well-known cause.
Kidney infections (pyelonephritis), low potassium levels, Addison’s disease (the opposite of Cushing’s, where the adrenal glands produce too little hormone), and leptospirosis, a bacterial infection, all belong on the list. Even a very low protein diet can occasionally contribute.
Medications That Trigger PU/PD
Some of the most commonly prescribed veterinary medications cause increased thirst and urination as a predictable side effect. Steroids like prednisone are the biggest culprit. If your dog recently started a steroid course and suddenly can’t stop drinking, the medication is almost certainly the reason. Certain seizure medications, diuretics, and thyroid supplements can also cause it. If PU/PD appears shortly after starting a new medication, your vet needs to know, because distinguishing a drug side effect from a new disease saves everyone time and worry.
Psychogenic Polydipsia
In rare cases, the excessive drinking has no physical cause at all. Psychogenic polydipsia is a behavioral condition where a dog compulsively drinks water due to anxiety, boredom, or environmental stress. Most dogs diagnosed with it are young, otherwise healthy, and often display other signs of nervousness or compulsive behavior like pacing, tail chasing, or hyperactivity. The kidneys work fine in these dogs. They’re simply overwhelmed by the volume of water coming in.
This diagnosis is always a last resort. Your vet will rule out every medical cause before considering it, because it’s far less common than the physical conditions that produce the same symptoms.
How to Track Your Dog’s Water Intake
Before your vet appointment, you can provide genuinely useful data by measuring how much your dog drinks over 24 hours. Use a single water bowl and fill it with a measured amount each morning. At the same time the next day, measure what’s left. If you have multiple pets, you’ll need to separate them during the measurement period for accuracy, or note that the number includes shared access. For a rough benchmark, multiply your dog’s weight in kilograms by 100. If the daily intake exceeds that number in milliliters, the increase is definitive. A 20 kg (44 lb) dog drinking more than 2 liters a day, for instance, is clearly in PU/PD territory.
Keep in mind that wet food contributes water while dry kibble doesn’t, so dogs eating exclusively dry food will naturally drink more from their bowl. Hot weather and heavy exercise also increase normal intake. What your vet is looking for is a sustained pattern, not a single hot afternoon.
What Happens at the Vet
Diagnosis typically starts with blood work and a urinalysis. The urine test is especially informative. Urine specific gravity measures how concentrated the urine is. Healthy kidneys produce urine that varies in concentration depending on hydration, but a specific gravity stuck around 1.010 regardless of how much the dog has had to drink suggests the kidneys have lost significant function. A reading consistently below 1.010 points toward problems with the hormone that controls water reabsorption, such as diabetes insipidus or psychogenic polydipsia.
Blood work reveals kidney values, blood sugar, calcium levels, liver enzymes, and electrolyte balance, which together help narrow the list considerably. If Cushing’s disease is suspected, your vet will recommend specific hormone tests. In some cases, imaging like an abdominal ultrasound helps visualize the kidneys, adrenal glands, or liver.
The diagnostic process can take more than one visit. PU/PD has so many possible causes that vets work through them systematically, starting with the most common and working toward the rare. The initial blood panel and urine test usually eliminate a large portion of the list right away, giving your vet a clear direction for the next step.