For dogs, the standard “rule of thumb” water intake is:

Normal Daily Water Intake

About:

40-60 mL/kg/day

Some internists use:

• 50 mL/kg/day as the mental midpoint

Because veterinarians love round numbers almost as much as we love pretending the owner can accurately measure water intake in a three-dog household featuring one Labrador and a toilet-drinker.

Quick Examples

10 kg dog

Normal:

• about 400-600 mL/day

20 kg dog

Normal:

• about 800-1200 mL/day

30 kg dog

Normal:

• about 1.2-1.8 liters/day

When Do We Call It Polydipsia?

Generally:

>90-100 mL/kg/day

raises concern for true polydipsia.

That’s the classic threshold.

So:

• 20 kg dog drinking >2 liters/day
  starts becoming suspicious.

Important Caveats

Water intake varies with:

• canned vs dry food
• exercise
• ambient temperature
• panting
• lactation
• fever
• steroid use
• sodium intake

A dog eating canned food may drink surprisingly little because they’re already getting water in the diet.

Meanwhile a husky running around South Carolina in July may drink like a malfunctioning fire hydrant and still be normal.

Clinically Useful Shortcut

Many clinicians mentally divide it like this:

Urine Production Rule of Thumb

Normal urine output:

• roughly 1-2 mL/kg/hour

Polyuria:

• often >4-5 mL/kg/hour

One Very Helpful Practical Pearl

Owners often overestimate drinking.

A lot.

Because:

• multiple pets
• spilled water
• evaporation
• refilling habits
• “he seems thirsty”

Actual measurement over 2-3 days is gold-standard useful.

Measured intake plus:

• USG
• serum sodium
• chemistry panel

usually gets you much closer to the truth.

Tiny internal medicine detective work. Everyone standing around a urine sample pretending the kidneys aren’t running the whole show.

Reasons a Dog Might Have a HYPOSTHENURIC URINE

(This dog has a USPG of 1.002)

Yes, Doc Jay. Historically, there absolutely were therapeutic/functional tests used to sort out causes of hyposthenuria and PU/PD. Some are still used carefully. Some have fallen out of favor because they can be dangerous if done cavalierly. Which, naturally, humans did for decades anyway.

The classic approaches are:

1. Desmopressin (DDAVP) Trial

This is the safest and most clinically useful “therapeutic diagnosis” now.

Used to evaluate for:

• Central Diabetes Insipidus
  vs
• nephrogenic DI
  vs
• psychogenic polydipsia

Principle

Desmopressin is synthetic ADH.

If the animal lacks ADH:

• urine concentrates dramatically after DDAVP

If kidneys cannot respond:

• little or no response

Expected Responses

Central DI

Big response:

• water intake decreases
• urine specific gravity rises

Often dramatic.

Example:

• USG 1.002 → 1.020+

Nephrogenic DI

Minimal or no response.

Because the kidney is ignoring ADH already.

Psychogenic Polydipsia

Variable/mild response.

These patients already have ADH.
The issue is excessive water intake and medullary washout.

Sometimes after several days of controlled water intake plus DDAVP, concentrating ability improves somewhat.

Practical Veterinary Method

Typical dog/cat:

• ophthalmic DDAVP drops in conjunctiva
  or
• intranasal solution orally/conjunctivally

Monitor:

• water intake
• urine specific gravity

over several days.

2. Water Deprivation Test

Classic. Powerful. Potentially dangerous. The endocrine equivalent of:

“Let’s see whether dehydration kills the patient before the kidneys decide to cooperate.”

This is NOT something to do casually.

Principle

Normally:

• dehydration → ADH release → concentrated urine

If urine stays hyposthenuric despite dehydration:

• DI becomes likely

Interpretation

Primary Polydipsia

Eventually concentrates urine.

Central DI

Fails initially, then improves after DDAVP.

Nephrogenic DI

Fails even after DDAVP.

Why Dangerous?

If the patient truly has DI:

• severe hypernatremia
• neurologic injury
• collapse

can occur.

So modern medicine often prefers:

• imaging
• baseline labs
• DDAVP trial

before formal deprivation testing.

3. Salt Loading

Historically discussed more in physiology than practical veterinary medicine.

You mentioned feeding salt. Intellectually reasonable. Clinically messy.

Why it’s unreliable

Salt should:

• increase plasma osmolality
• stimulate endogenous ADH

If urine still remains hyposthenuric:

• impaired ADH pathway suspected

BUT:

• risky
• poorly standardized
• confounded by drinking behavior
• can worsen hypernatremia

So it’s generally not a preferred diagnostic approach.

4. Controlled Water Restriction / Medullary Washout Recovery

This is sometimes clinically informative.

If chronic psychogenic polydipsia caused medullary washout:

• gradual normalization of water intake
• monitored restriction
• time

may restore concentrating ability over days.

Urine specific gravity slowly rises.

That suggests:

• kidneys can respond
• concentrating gradient had been washed out

5. Measure Osmolality

Modern workup often includes:

• serum osmolality
• urine osmolality

because USG alone is imperfect.

Patterns help distinguish:

• DI
• psychogenic polydipsia
• renal disease

Important Clinical Point

Before chasing DI aggressively, you rule out the common offenders first:

• hypercalcemia
• hypokalemia
• renal disease
• pyometra
• hyperadrenocorticism
• liver disease
• drugs

Because true DI is relatively uncommon.

Meanwhile Cushing’s, steroids, psychogenic drinking, and medullary washout are out there everywhere causing chaos like raccoons in a campground.

And one more thing, Sir:
If a patient is making truly hyposthenuric urine consistently, that actually tells you the kidneys retain some tubular function. They are actively diluting. That’s diagnostically useful and somewhat reassuring compared with dead-flat isosthenuria.