Polyuria in dogs is the medical term for producing an abnormally large volume of urine, defined as more than 50 ml per kilogram of body weight per day. For a 20 kg (44 lb) dog, that means producing over 1 liter of urine daily. Normal urine output falls between 20 and 40 ml/kg per day, so a dog with polyuria is urinating at least 25% more than the upper end of normal. It almost always shows up alongside increased thirst, a paired condition vets refer to as PU/PD (polyuria/polydipsia).
How Normal Urine Production Works
A dog’s kidneys filter blood continuously, producing a large volume of dilute fluid that then gets concentrated as it passes through tiny tubes called nephrons. The final step happens in the collecting ducts, where a hormone called ADH (antidiuretic hormone) signals the kidneys to pull water back into the body. This process is what keeps urine concentrated and volume low. When something disrupts ADH production, blocks its effect on the kidneys, or overwhelms the kidneys with substances they need to flush out, the result is polyuria.
There are two basic ways this goes wrong. In one pattern, excess dissolved substances like glucose or urea pull water into the urine by osmotic force, increasing volume even though the kidneys are working normally. In the other, the kidneys lose their ability to respond to ADH and can’t concentrate urine properly. Excess cortisol, for example, interferes with ADH at the collecting duct. Research on cortisol-treated dogs showed urine concentration dropped from around 897 to just 186 mosm/kg, while urine production jumped from 0.7 to 9.3 ml/kg per hour. About two-thirds of that increase came from the kidneys simply failing to reclaim water.
The Three Most Common Causes
Dozens of conditions can trigger polyuria, but in dogs, three account for the majority of cases: kidney disease, Cushing’s disease (overproduction of cortisol), and diabetes mellitus.
Kidney Disease
As kidneys lose functional tissue, they can no longer concentrate urine effectively. Dogs in early kidney failure often show increased urination and thirst as the first noticeable signs, sometimes months before they seem sick in other ways. The urine becomes progressively more dilute because fewer nephrons are available to do the work of reabsorbing water.
Cushing’s Disease
Dogs with Cushing’s disease produce too much cortisol, typically because of a small tumor on the pituitary gland or, less commonly, on an adrenal gland. The excess cortisol directly blocks the kidneys’ ability to respond to ADH, causing large volumes of dilute urine. Increased thirst, a pot-bellied appearance, hair thinning, and panting are other hallmarks. Middle-aged and older dogs, especially breeds like Poodles, Dachshunds, and terriers, are most commonly affected.
Diabetes Mellitus
When a dog can’t produce enough insulin or can’t use it properly, glucose builds up in the blood. Once blood glucose exceeds the kidneys’ ability to reabsorb it, the excess spills into the urine and drags water along with it through osmotic force. The result is high-volume, sugar-containing urine. Weight loss and increased appetite typically accompany the excessive urination and drinking.
Other Conditions That Cause Polyuria
Beyond the big three, a long list of less common conditions can produce the same symptom. Liver failure and portosystemic shunts (abnormal blood vessels that bypass the liver) impair the organ’s ability to process waste, which affects kidney concentration. Elevated blood calcium from cancer or other causes directly interferes with the kidneys’ concentrating mechanism. Uterine infections (pyometra) in unspayed females release bacterial toxins that block ADH activity, making sudden increased urination in an intact female dog a potential emergency sign.
Diabetes insipidus, a rare condition unrelated to blood sugar, comes in two forms. In the central type, the brain doesn’t produce enough ADH. In the nephrogenic type, the kidneys ignore the ADH that’s there. Both lead to massive urine volumes, sometimes dramatically more than what’s seen with other causes. Addison’s disease (low cortisol, the opposite of Cushing’s) can also cause polyuria through sodium imbalances, along with low potassium, certain kidney infections, and very low-protein diets.
Medications That Trigger Polyuria
If your dog recently started a new medication and is suddenly urinating far more than usual, the drug itself may be the cause. Steroids (glucocorticoids like prednisolone) are the most common culprits. They’re widely prescribed for allergies, inflammation, and immune conditions, and polyuria with increased thirst is their most frequent side effect. The mechanism is the same as Cushing’s disease: the steroid blocks ADH action in the kidneys.
Other medications known to cause polyuria include diuretics (prescribed for heart conditions), anticonvulsants like phenobarbital (used for seizures), thyroid hormone supplements, and salt supplementation. If the timing lines up with starting one of these drugs, that’s important information for your vet.
Psychogenic Polydipsia
Occasionally, the problem starts with drinking rather than urinating. Some dogs develop a behavioral pattern of compulsive water consumption, and the excess urination is simply the body’s way of getting rid of the extra fluid. This is called psychogenic polydipsia, and it’s considered a diagnosis of exclusion, meaning every other cause has to be ruled out first. There is no single test that confirms it. Vets look for a documented behavioral component, such as anxiety, boredom, or compulsive tendencies, alongside normal bloodwork and kidney function. Some cases get labeled psychogenic simply because no medical explanation can be found, even without clear behavioral abnormalities.
How Vets Diagnose the Cause
The good news is that most causes of polyuria can be identified with a straightforward initial workup. A thorough history (when the symptoms started, any new medications, changes in appetite or weight), a physical exam, a complete blood count, a blood chemistry panel, and a urinalysis with bacterial culture will catch the majority of underlying conditions.
Urine concentration, measured as specific gravity, is one of the most useful early clues. In a healthy, normally hydrated dog, urine specific gravity is typically 1.030 or higher. Dogs with polyuria often produce urine well below that threshold. Very dilute urine, especially in a dehydrated dog, tells the vet the kidneys aren’t concentrating properly and points toward specific categories of disease. The blood chemistry panel can reveal elevated blood sugar (diabetes), elevated kidney values, high calcium, liver enzyme changes, or cortisol-related patterns that narrow the list further.
If the initial workup doesn’t reveal a clear answer, more targeted testing follows. This might include cortisol-specific blood tests for Cushing’s disease, imaging of the abdomen to evaluate kidney or adrenal gland size, or hormone trials to differentiate between types of diabetes insipidus.
What to Watch for at Home
Measuring your dog’s exact urine output is difficult outside a veterinary setting, but you can track water intake, which is a practical proxy. Normal water consumption for dogs is roughly 50 to 70 ml per kilogram of body weight per day, depending on diet, activity, and weather. For a 20 kg dog, that’s about 1 to 1.4 liters. Dogs eating wet food drink less because their food already contains significant moisture; dogs eating only dry kibble tend to drink at the higher end of the range.
To get a rough measurement, fill your dog’s water bowl with a known amount each morning and measure what’s left at the end of the day, accounting for any other water sources. If intake consistently exceeds 100 ml/kg per day (2 liters for that same 20 kg dog), that crosses the clinical threshold for polydipsia and warrants investigation. Other signs to note include needing to go outside more frequently, having accidents in the house (especially in a previously house-trained dog), producing large puddles of pale or nearly colorless urine, and waking you up at night to go out.
Because polyuria is a symptom rather than a disease, the treatment depends entirely on what’s driving it. Kidney disease, Cushing’s, and diabetes each have distinct management paths, and medication-induced polyuria often resolves when the drug is adjusted or discontinued. The key step is identifying the pattern early and getting the initial bloodwork and urinalysis done, which will clarify the direction for most dogs.