Many people bring their dogs in because they've noticed either increased drinking or increased peeing.

Even if only one is reported, the problems go together. Drinking more causes peeing more and peeing more causes increased thirst.

The first thing we need to do is confirm that the drinking and urinating is actually excessive.

 
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The basics

How much water is too much?

Excessive water consumption is called polydipsia (PD).

The amount of water any individual drinks varies from day to day depending on things like hot weather, exercise and different foods.

  • Normal water consumption is usually <60 ml/kg/day (ie <600 ml for a 10 kg dog)

  • Polydipsia is confirmed if your dog} is drinking more than 100 ml/kg/day (ie >1 L for a 10 kg dog)

  • Between 60–100 ml/kg/day is in a grey zone and we need to consider whether this represents a significant increase from the previous 'normal' water consumption and other factors

To know how much your dog is drinking, you'll need to measure the amount of water he drinks over a 24-hour period.

 

How much pee production is too much?

Production of an excessive amount of urine is called polyuria (PU). This is not the same as doing lots of little pees (pollakiuria) or urinary incontinence.

While urine amounts are obviously very much harder to measure at home, there are levels considered normal and abnormal:

  • Normal urine output is <50 ml/kg/day

  • Polyuria is defined as >50 ml/kg/day

Because we can't easily measure urine output, we use the concentration of the urine to help us determine the likeliness of polyuria. Persistently passing large amounts of dilute urine supports polyuria.

As polyuria and polydipsia go together, we tend to use the abbreviation of PU/PD even when a patient is brought in with only one of the two reported.

 
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What causes PU/PD?

Once we've confirmed that we're dealing with true PU/PD we need to work out why it's occurring. There are lots of possible causes but only three different mechanisms that lead to PU/PD: primary polydipsia, osmotic diuresis and a failure of urine concentration.

Some of the causes of PU/PD are listed here under the different mechanisms (note that this is simplified and in reality, many of these diseases fit into more than one category). The most common ones are in bold.

 

Primary polydipsia

(Where a dog drinks a lot causing excessive urine production)

  • Psychogenic polydipsia – which is a rare behavioural disorder that causes primary PD. We don’t know why it occurs

 

Osmotic diuresis

(Due to molecules present in urine that draw water out of the body)

  • Diabetes mellitus

  • Primary renal glucosuria

 

Failure to concentrate urine

(Due to multiple different mechanisms that interfere with the kidneys' ability to conserve water)

  • Renal dysfunction

    • chronic kidney disease

    • polyuric stage of acute kidney disease

    • post-obstructive diuresis

    • renal medullary washout


  • Primary nephrogenic diabetes insipidus – which is a congenital disorder where the kidneys don’t respond to antidiuretic hormone (ADH)


  • Secondary nephrogenic diabetes insipidus (acquired)

    • pyometra (uterine infection)

    • pyelonephritis (kidney infection)

    • hypercalcaemia (often due to cancer)

    • liver disease

    • portosystemic shunt

    • hyperadrenocorticism (Cushing's)

    • hypoadrenocorticism (Addison's)

    • hyperaldosteronism (Conn's)

    • hypokalaemia

    • polycythaemia


  • Central diabetes insipidus – which is a condition of inadequate production of ADH

    • congenital

    • acquired (pituitary disease)


  • Drugs

    • glucocorticoids

    • phenobarbitone

    • diuretics

    • salt supplementation

 

Even if we only consider the more common causes, there are still multiple diseases to consider. This means that it's rare we can give you a simple diagnosis during a consultation about why your dog has PU/PD.

 
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How do we investigate PU/PD?

History

Along with confirming the presence of PU/PD, we'll be looking for clues in history you give that can help narrow down the list of possible causes. For example, a dog with a ravenous appetite is more likely to have diabetes mellitus or Cushing's disease than renal failure.


Physical examination

Again, we're looking for clues to help us narrow the list down. For example, enlarged lymph nodes could mean lymphoma, which is associated with elevated calcium levels in the blood (hypercalcaemia).


Initial testing

We typically start with what's called a minimum database that includes:

  • haematology (red and white blood cells)

  • biochemistry

  • urine testing

A minimum database allows us to diagnose or rule out many of the most common causes of PU/PD such as diabetes mellitus, acute/chronic kidney disease and hypercalcaemia. If no diagnosis has been reached, additional testing is needed.


Further testing

Depending on the results of the minimum database, we may recommend:

  • further blood testing (eg liver function test, cortisol testing)

  • imaging (ultrasound and X-ray)

If these tests are also normal, we can narrow the differential diagnosis to diabetes insipidus (central or nephrogenic) and primary polydipsia. To work out which one of these we're dealing with, we have two potential tests that tell us whether it's possible for {AnimalName} to concentrate urine:

  • a trial with a drug called vasopressin (or desmopressin)

  • a modified water deprivation test

If we get to the stage that we need to consider these, we'll discuss the pros and cons of each before deciding which direction to take.