Polyuria and polydipsia means over 50 mL of water per kilogram per day going in, and over 50 mL of urine per kilogram per day coming out. A 4 kg cat tops out near 240 mL in the bowl, so a bowl that drops 350 mL in 24 hours is a case rather than a habit. Owners notice the litter box first: 6 or 8 clumps where there used to be 3.
Two tests sort most of it, and neither needs a specialist: a urine sample with specific gravity, and a blood panel with glucose, T4, kidney values and potassium.
Where normal ends and PU/PD begins
A healthy cat drinks 40 to 60 mL/kg a day and takes another 15 to 25 mL/kg from wet food, which is 70 to 80 percent water. Urine output runs 20 to 30 mL/kg, which at 4 kg appears as 3 to 5 clumps of 30 to 60 g. Measure with a jug against a marked level for 7 days, because one day moves with room temperature and with how much dry food is out.
Urine specific gravity (USG) is the cheapest discriminator. Normal is 1.035 to 1.060. A cat with true PU/PD cannot concentrate above 1.035, and 1.020 or lower, close to the 1.000 of plasma, is evidence rather than an impression. One dilute sample in a well cat is worth repeating in 48 hours; two earn a workup.
Three groups of causes
Nearly every case lands in one of three buckets, and the bucket decides the next test.
- Kidney. Chronic kidney disease (CKD) is the most common cause after age 7, and thirst rises once about two thirds of function is gone. Pyelonephritis, the 24 to 72 hours of diuresis after a blocked bladder is relieved, and renal dysplasia in a young cat belong here.
- Endocrine. Diabetes mellitus with glucose above 250 mg/dL and sugar in the urine; hyperthyroidism, usually T4 above 4.0 µg/dL with weight loss and a ravenous appetite; hyperadrenocorticism, uncommon but real with a pot belly and thin skin.
- Osmotic and other. Calcium above 12 mg/dL, potassium below 3.0 mmol/L, liver disease, pyometra in an entire female, corticosteroid treatment such as prednisolone, and primary polydipsia, a diagnosis of exclusion that needs 3 weeks of records first.
Age narrows it fast. Under 6 years, think diabetes, dysplasia, psychogenic drinking and steroids. Over 8, CKD and hyperthyroidism cover most of the list, and both are cheap enough to test on the first visit.
The two tests that split the groups
Test one is urine: USG, glucose, ketones, protein, sediment, and a culture if white cells are present. A free-catch sample is fine for chemistry, but culture needs a cystocentesis sample and a bladder large enough to be safe.
Test two is blood: glucose, T4, creatinine, SDMA, urea, potassium, calcium, phosphate and a CBC. SDMA rises at about 40 percent function loss where creatinine waits for 75 percent, which is why it belongs in a first panel and not a second one.
If both come back normal and the cat still drinks 300 mL a day, the next steps are imaging, an ACTH stimulation test, and a water-deprivation trial run in hospital over 6 to 12 hours, never at home.
The first 24 hours at home
Hour 0 to 2: put out a measured bowl, note the time, and stop estimating. Hour 2 to 6: count clumps and weigh them on a kitchen scale, because 1 g of clump is close to 1 mL of urine. Hour 6 to 12: take a rectal temperature, look at the gums, lift the skin at the shoulders. Hour 12 to 18: photograph the box and the bowl. Hour 18 to 24: total both numbers and compare with 50 mL/kg.
Water stays out the whole time. Cutting water in a cat already losing it through the kidneys concentrates the urine further and can tip a borderline cat into dehydration inside 12 hours. If there is soft or yellow stool as well, the gut is a fourth line of enquiry, covered in yellow diarrhea in cats.
Triage: how many hours you have
These are the numbers that change the deadline. A cat that is merely drinking a lot can wait for a weekday appointment; a cat that cannot pass urine cannot wait for the clinic to open.
| What you see | Number that matters | Deadline |
|---|---|---|
| Straining in the box, no urine | 0 mL for 6 hours | Now, within 1 hour |
| Vomiting, refusing food | No food for 24 hours, 3 vomits in 12 | Within 6 hours |
| High glucose plus ketones | Glucose above 250 mg/dL | Within 2 to 4 hours |
| Cold, hidden, weak | Temperature below 37.5 C | Within 1 hour |
| Drinking 400 mL, losing weight | Above 50 mL/kg/day, 5 percent loss in 8 weeks | Within 48 hours |
| Dilute urine, cat still bright | USG 1.035 or lower | Within 72 hours |
| New PU/PD over age 8 | 300 mL a day at 4 kg | Within 7 days |
What the record has to contain
Seven days, four columns, one page. Without it the vet is guessing and so are you.
- Date and millilitres poured in 24 hours, from the jug rather than the bowl.
- Clumps: count and total weight in grams, converted at 1 g to 1 mL.
- Body weight weekly, same scale, before breakfast.
- Appetite scored 0 to 3, plus urine colour and any blood or crystals.
Bring the sheet, a photo of the box, and the food label: a can stating 80 percent moisture changes the water maths, and it is the detail most often left at home.
What not to do
Do not start a kidney diet before a diagnosis, because lower protein and phosphate muddy the next panel. Do not restrict water at any point. Do not write it off as old age, since a cat of 12 with PU/PD deserves the same two tests as a cat of 4. Do not use leftover antibiotics, which wreck the culture you are about to need. Do not assume the windowsill plant is harmless: lily and antifreeze exposure both cause thirst and vomiting, and the toxicity checker takes 2 minutes.
Five steps this week
- Measure 24 hours of intake with a jug and weigh that day's clumps.
- Catch urine in a box of non-absorbent beads, and refrigerate it for up to 12 hours if the visit is tomorrow.
- Book urine and blood for the same day, and ask for USG, glucose, T4, SDMA and potassium by name.
- Weigh weekly and keep the four-column record for 2 weeks after the visit.
- Put water in 3 places and add 1 wet meal a day, which brings 70 to 100 mL without forcing anything.
Terms mentioned in this article
Frequently asked
How much water is too much for a cat?
More than 50 mL per kilogram per day is the line for polydipsia, so a 4 kg cat drinking over 200 mL is at the top of normal and 300 mL or more needs a workup. Pair the number with urine specific gravity: a reading below 1.035 in a cat that is not on wet food or steroids supports true polyuria rather than a big drinker.
Can I just restrict water to stop the peeing?
No. In a cat losing water through the kidneys, restriction concentrates urine further and can cause dehydration within 12 hours, sometimes within 6. Water stays available at all times, and the fix is a diagnosis: urinalysis with specific gravity and a blood panel with glucose, T4, SDMA and potassium on the same day.
Which causes are most likely at 10 years old?
Chronic kidney disease and hyperthyroidism cover most older cats, with diabetes third. CKD usually shows a USG at or below 1.035, a creatinine above 1.6 mg/dL and a rising SDMA, while hyperthyroidism shows a T4 above 4.0 µg/dL with weight loss despite a good appetite. Both are cheap to test and both change management within weeks.