When Thirst Becomes a Symptom
Dogs drink more water in hot weather, after exercise, and when fed a dry food diet. These fluctuations are entirely normal. But when a dog is consistently drinking and urinating far more than usual — regardless of temperature, activity level, or diet — it warrants investigation. The clinical term for excessive thirst is polydipsia, and it nearly always accompanies polyuria, meaning excessive urine production. The two are so consistently linked that vets refer to the combination as PU/PD.
A rough guide: dogs typically drink between 20 and 70 millilitres of water per kilogram of body weight per day. Intake consistently above 100ml per kg per day is considered excessive. If you find yourself refilling the water bowl far more than usual, or your dog is waking in the night to drink, it is worth making a note and booking a veterinary appointment.
How the Body Normally Regulates Thirst and Urination
Understanding why polydipsia occurs requires a brief look at how the body normally controls fluid balance. The hypothalamus in the brain monitors blood osmolality — essentially the concentration of solutes in the bloodstream. When blood becomes too concentrated, the hypothalamus triggers thirst and signals the pituitary gland to release antidiuretic hormone (ADH), also known as vasopressin. ADH acts on the kidneys, causing them to reabsorb water and produce more concentrated urine.
When any part of this system is disrupted — whether by hormonal imbalance, kidney dysfunction, or high blood glucose — the result is dilute, voluminous urine and a compensatory increase in drinking.
The Most Common Causes
Diabetes mellitus
Diabetes is one of the most frequently diagnosed causes of PU/PD in dogs. When blood glucose rises beyond the kidney's capacity to reabsorb it, glucose spills into the urine and draws water with it through osmosis — resulting in large volumes of dilute urine and intense thirst. Affected dogs often also show weight loss despite a good or increased appetite. Middle-aged, neutered female dogs and certain breeds including Samoyeds, Poodles, and Pugs are at elevated risk.
Cushing's disease (hyperadrenocorticism)
Cushing's disease results from chronically elevated cortisol, usually due to a small tumour on the pituitary gland. Cortisol antagonises ADH, reducing the kidneys' ability to concentrate urine. The result is dramatically increased water intake and urination. Dogs with Cushing's often also develop a pot-bellied appearance, thinning skin, hair loss, and muscle wasting. It is particularly common in older dogs and in breeds such as Boxers, Dachshunds, and Poodles.
Chronic kidney disease
As kidney tissue is progressively lost, the remaining nephrons lose their ability to concentrate urine effectively. The kidneys must produce larger volumes of dilute urine to excrete the same amount of waste, driving increased thirst to compensate. PU/PD is often one of the first observable signs of CKD, appearing well before clinical illness becomes obvious.
Pyometra
In unspayed female dogs, pyometra — a serious uterine infection — is a common and life-threatening cause of increased thirst. Toxins produced by the infection interfere with ADH receptors in the kidneys. Pyometra is a surgical emergency, but once the uterus is removed the PU/PD typically resolves.
Hypercalcaemia
Abnormally high blood calcium levels can result from a range of causes including certain cancers, primary hyperparathyroidism, vitamin D toxicity, and Addison's disease. Calcium interferes with ADH signalling in the kidney, impairing urine concentration. It is a less common but important cause to rule out, particularly in dogs with concurrent weight loss or other systemic signs.
Diabetes insipidus
A less common condition, diabetes insipidus involves either a deficiency of ADH (central DI) or the kidneys' inability to respond to it (nephrogenic DI). The result is production of extremely large volumes of very dilute urine and intense, sometimes dramatic thirst. Unlike diabetes mellitus, blood glucose is normal. Central DI may follow head trauma or result from a pituitary tumour.
Liver disease
Chronic liver disease and hepatic encephalopathy can disrupt the mechanisms that normally generate urine concentration in the kidney. The relationship is complex and involves impaired production of urea within the kidney medulla. Dogs with liver disease often show other signs including jaundice, ascites, and altered mentation.
Medications
Several commonly prescribed medications cause PU/PD as a side effect. Corticosteroids are the most frequently encountered cause — even short courses of prednisolone can dramatically increase thirst and urination. Diuretics, certain anti-seizure medications, and salt supplementation will also increase water intake. Always inform your vet of any medications or supplements your dog is receiving.
How Vets Investigate PU/PD
A systematic approach is required because the list of differentials is broad. Your vet will typically begin with a full history, a physical examination, and baseline diagnostics.
Initial tests usually include
- Complete blood count and serum biochemistry panel — assessing kidney function, liver values, blood glucose, and calcium
- Urinalysis with culture — checking urine concentration and ruling out infection
- Urine specific gravity — a key indicator of the kidney's ability to concentrate urine
From there, further testing is guided by the initial findings. This might include cortisol testing, an ACTH stimulation test for Cushing's, abdominal ultrasound, or a water deprivation test (performed under close veterinary supervision) to distinguish between different causes of dilute urine.
Why You Should Not Wait to See If It Resolves
Polydipsia and polyuria are almost always a sign that something physiologically significant is happening. Unlike vomiting or an upset stomach, they rarely resolve on their own. Many of the underlying causes — kidney disease, diabetes, Cushing's, pyometra — are progressive conditions where early diagnosis genuinely improves outcomes. The diagnostic process is usually straightforward, and identifying the cause quickly means treatment can begin before significant damage occurs.