Veterinary CPD webinar – Understanding Polyuria–Polydipsia (PU/PD) in Companion Animals

What Is PU/PD?

PU/PD is considered a syndrome, meaning it consists of more than one clinical sign, specifically excessive urination and excessive drinking. These signs typically develop gradually, which is why owners may delay seeking veterinary attention. However, early recognition and a structured diagnostic approach are essential.

Step One: Confirm the Problem

Before assuming disease, PU/PD must be objectively confirmed:

• Measure water intake over 24 hours (owner homework)
• Alternatively, assess urine output, though this is usually impractical outside of a hospital setting

In daily practice, urine specific gravity (USG) is the most useful surrogate marker.

• Normal USG:
  • Cats: >1.035
  • Dogs: >1.030

USG provides insight into renal concentrating ability and antidiuretic hormone (ADH) activity, though it does not identify the exact cause of dysfunction.

The Importance of a Thorough History

A detailed history is critical and should include:

• Age (congenital vs acquired disease)
• Environment and activity level
• Diet (including home-cooked, raw, or high-salt diets)
• Medications and supplements (e.g. steroids, diuretics, phenobarbital)
• Reproductive status (e.g. intact females and pyometra)
• Behavioural factors

Many cases can be explained, or ruled out, at this stage alone.

Physical Examination and Baseline Testing

A full physical exam is non-negotiable and should include oral, rectal, vaginal, skin, and neurological assessments.

Baseline diagnostics should include:

• Full blood panel with electrolytes
• Thyroxine (TT4)
• Complete urinalysis
• Sterile urine culture (sediment alone is not sufficient)

These tests help identify common causes such as diabetes mellitus, kidney disease, infections, or endocrine disorders.

Understanding the Physiology Behind PU/PD

Fluid balance is regulated by two key systems:

• Central system – hypothalamus and pituitary (ADH/vasopressin production)
• Peripheral system – kidneys and vascular volume

ADH plays a central role in urine concentration. Disruption can occur due to:

• Brain disease
• Renal tubular dysfunction
• Osmotic diuresis
• Medullary washout
• Hormonal and metabolic disease

Triggers for increased drinking may also include pain, nausea, stress, exercise, dry food, or medications.

Key Differential Diagnoses

Some of the most important causes of PU/PD include:

• Diabetes mellitus
• Diabetes insipidus (central or nephrogenic)
• Hyperadrenocorticism (Cushing’s)
• Pyometra
• Kidney disease
• Hypercalcemia
• Liver disease
• Leptospirosis
• Drug-induced PU/PD

Notably, secondary nephrogenic diabetes insipidus is one of the most common mechanisms underlying PU/PD and is associated with many everyday diseases.

Renal Medullary Washout

Chronic excessive water intake can reduce the kidney’s ability to concentrate urine, a process known as renal medullary washout.
Management requires:

• Gradual, controlled water restriction
• Close monitoring of renal parameters
• Time for the medullary concentration gradient to re-establish

Diagnostic Strategy: The “Onion” Approach

PU/PD should be investigated layer by layer:

• Confirm true PU/PD
• Rule out common systemic causes
• Assess renal concentrating ability
• Evaluate central vs nephrogenic causes
• Consider behavioural (psychogenic) polydipsia

Water deprivation testing is no longer recommended due to animal welfare concerns. Instead, a carefully monitored desmopressin trial, combined with gradual water restriction, is preferred.

Treatment and Prognosis

Treatment depends entirely on the underlying cause:

• Central diabetes insipidus often responds well to desmopressin
• Secondary causes require management of the primary disease
• Some cases may not require treatment if the animal’s quality of life is unaffected

Prognosis varies widely and is dictated by the root pathology rather than the PU/PD itself.

Key Takeaways

• PU/PD is multisystemic, not just a urinary tract issue
• History and basic diagnostics are incredibly powerful
• Most cases are due to primary polyuria, not primary polydipsia
• A structured, stepwise approach prevents missed diagnoses
• Always treat the cause, not just the symptom

By methodically “peeling back the layers,” clinicians can confidently navigate even the most complex PU/PD cases.

If you are thinking about developing your career and looking for a new role in a practice that will support your development and CPD time, why not schedule a chat with one of our consultants? They can advise you on your career options and help you achieve your long-term career goals.