Polydipsia: leading causes in cats and dogs

Causes of polydipsia

The main causes of polydipsia include:

• Diabetes mellitus: insulin deficiency results in hyperglycaemia, which is in turn responsible for glycosuria and osmotic diuresis.
• Hyperadrenocorticism (Cushing’s syndrome): glucocorticoids can both inhibit the release of vasopressin and antagonise its action.
• Hypoadrenocorticism:the loss of sodium leads to a reduction in medullary hypertonicity.
• Hyperthyroidism: thyroid hormones have a diuretic effect that causes increased renal blood flow and decreased medullary hypertonicity.
• Acute end-stage renal failure:  osmotic diuresis results from the retention of solutes during the oliguric phase.
• Chronic renal failure: if there is a marked decrease in the number of functioning nephrons, then the remaining nephrons are overloaded with solutes, causing osmotic diuresis.
• Pyelonephritis: this results in increased renal blood flow and reduced medullary hypertonicity. Tubular necrosis caused by bacteria may also contribute to this.

Diagnosis

The first step in the diagnosis of polydipsia is to rule out an iatrogenic cause by examining the patient’s clinical history.

Once this has been excluded, laboratory tests with a complete blood count and chemistry, urinalysis and a thyroid hormone T4 assay should be performed in the case of cats.

If it is still impossible to reach an aetiological diagnosis despite these complementary tests, extend the test battery with:

• An abdominal ultrasound
• An adrenal function test
• The bile acid stimulation test
• A urine culture
• A GFR test (glomerular filtration rate)

If this battery of tests has still not confirmed a definitive diagnosis, there are only two remaining possibilities:

Central diabetes insipidus: the total or partial inability to produce or release desmopressin. This requires advanced neuroimaging tests as well as a desmopressin response test.

Primary polydipsia: identified by means of a water deprivation test.

Primary polydipsia

This is a very rare condition, in which compulsive water intake leads to compensatory polyuria. It generally affects dogs, especially very nervous or stressed females.

The only problem related to primary polydipsia is the inconvenience for owners if the dog lives predominantly indoors.

The treatment of primary polydipsia consists of gradually restricting water intake and above all action to address the trigger factor, which is usually stress. If the dog lives primarily outdoors, no treatment is needed.