PU/PD stands for polyuria/polydipsia, the medical shorthand veterinarians use when an animal is urinating more than normal and drinking more water than normal. These two signs almost always appear together because one drives the other: either the body produces too much urine and the animal drinks more to compensate, or the animal drinks excessively and the kidneys flush the extra fluid. PU/PD is not a diagnosis on its own but a clinical sign that points toward an underlying condition that needs investigation.
What Counts as Excessive Drinking and Urination
Veterinarians define these signs using specific thresholds based on body weight. For dogs, drinking more than 90 milliliters of water per kilogram of body weight in 24 hours qualifies as polydipsia. For cats, the threshold is lower: more than 45 milliliters per kilogram per day. To put that in perspective, a healthy 10-kilogram (22-pound) dog would normally drink less than about 900 milliliters (roughly 4 cups) of water a day. If that same dog starts consistently drinking 5 or 6 cups, something may be off.
On the urine side, dogs normally produce about 20 to 50 milliliters of urine per kilogram per day. Polyuria is defined as urine output exceeding 50 milliliters per kilogram per day. For cats, normal output is around 18 to 28 milliliters per kilogram per day, with polyuria beginning above 40 milliliters per kilogram per day. Most pet owners won’t measure urine volume directly, but they’ll notice the signs: a dog asking to go outside far more often, a cat filling the litter box faster, or accidents in a previously house-trained pet.
Why the Two Signs Go Together
The kidneys regulate how much water stays in the body versus how much leaves as urine. They do this largely through a hormone called antidiuretic hormone (ADH), which signals the kidneys to hold onto water. When this system works correctly, the body produces concentrated urine and maintains stable hydration. When something disrupts it, one of two things happens.
In most PU/PD cases, the primary problem is polyuria. The body produces too much dilute urine, either because the kidneys can’t respond to ADH, because ADH isn’t being produced in sufficient quantities, or because something else is overriding the system. The animal then gets thirsty and drinks more to replace what it’s losing. This is the more common pattern.
Less frequently, the primary problem is polydipsia. The animal drinks excessively first, often due to a behavioral or neurological issue, and the kidneys simply flush the surplus. This is called primary polydipsia and is relatively rare in pets compared to the conditions that drive urine production up first.
Common Conditions That Cause PU/PD
A wide range of diseases can trigger PU/PD, which is part of why veterinarians take this sign seriously. The most frequently identified causes differ somewhat between dogs and cats.
In Dogs
Chronic kidney disease is one of the most common culprits. As the kidneys lose function, they become unable to concentrate urine effectively, so the dog produces large volumes of dilute urine and drinks heavily to stay hydrated. Diabetes mellitus is another major cause. When blood sugar climbs high enough, excess glucose spills into the urine and pulls water along with it through a process called osmotic diuresis, dramatically increasing urine output.
Cushing’s disease (hyperadrenocorticism), where the body overproduces the stress hormone cortisol, is a particularly common cause in middle-aged and older dogs. Cortisol interferes with ADH’s ability to signal the kidneys, leading to dilute urine and heavy water intake. Pyometra, a serious uterine infection in unspayed females, can also cause PU/PD because toxins from the infection impair the kidneys’ concentrating ability. Liver disease, Addison’s disease (hypoadrenocorticism), and high blood calcium levels round out the list of frequent causes.
In Cats
Chronic kidney disease and diabetes mellitus are top causes in cats as well. Hyperthyroidism, an overactive thyroid gland that is extremely common in older cats, is another frequent driver. It increases blood flow through the kidneys and raises urine output. Liver problems, including portosystemic shunts (abnormal blood vessel connections that bypass the liver), urinary tract infections, and less commonly Cushing’s disease can also produce PU/PD in cats.
How Veterinarians Investigate PU/PD
Because so many different conditions can be responsible, your vet will typically start with a broad screening approach rather than jumping to a single test. The first step is usually a combination of blood work and a urinalysis. Blood tests check organ function (kidney values, liver values, blood sugar) and screen for electrolyte imbalances or signs of infection. The urinalysis is especially important because it reveals how well the kidneys are concentrating urine.
Urine concentration is measured by a value called urine specific gravity. In a healthy pet, urine specific gravity is relatively high, meaning the kidneys are doing their job of pulling water back into the body. In animals with PU/PD, the urine is often dilute. A specific gravity in the range of about 1.008 to 1.012 suggests the kidneys aren’t concentrating or diluting urine at all, a state called isosthenuria, which points toward kidney dysfunction or hormonal disruption.
Depending on what the initial tests reveal, your vet may recommend additional diagnostics. Thyroid hormone levels, cortisol tests, abdominal ultrasound, or urine cultures might follow. In rare cases where initial testing doesn’t point to a clear cause, a water deprivation test can help determine whether the problem lies with ADH production or the kidneys’ response to it. This test is done under close veterinary supervision because withholding water from an animal that’s losing excessive fluid can be dangerous.
What You Can Do at Home
If you suspect your pet is drinking or urinating more than usual, the single most useful thing you can do before the vet visit is measure water intake for a day or two. Fill the bowl to a marked level in the morning, refill as needed (measuring what you add), and record the total consumed in 24 hours. If you have multiple pets sharing a bowl, this gets trickier, but even a rough measurement helps your vet decide how urgently to pursue testing.
For cats, paying attention to litter box weight or how often you’re scooping clumps can give a rough sense of urine output. For dogs, noting how often they need to go outside, whether they’re having accidents, or whether they’re suddenly asking for water at unusual times all provides helpful detail.
PU/PD that appears suddenly tends to be more concerning than a gradual increase, though both warrant investigation. Many of the conditions behind PU/PD are very manageable when caught early. Diabetes can be controlled with insulin, thyroid disease with medication, and kidney disease can be slowed with dietary changes and supportive care. The abbreviation itself might sound clinical, but it describes something most pet owners can spot if they know what to look for: a pet that’s suddenly always at the water bowl and always needing to go out.