Marjorie L. Chandler, DVM, MS, MACVSc, DACVN, DACVIM, DECVIM, MRCVS
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Polyuria and polydipsia have many causes in dogs (Table 1). Obtaining a thorough history and performing a physical examination, a complete blood count, a serum chemistry profile,
a full urinalysis including urine bacterial culture, and imaging (e.g. ultrasonographic examination of the kidneys, liver, and adrenal glands) will help rule out many of the common causes.
Measuring the basal serum cortisol concentration may help rule out hypoadrenocorticism since this disorder is unlikely if
the value is > 2 μg/dl (~70 mmol/L). If the value is ≤ 2 μg/dl, hypoadrenocorticism must be confirmed with an ACTH stimulation
test. If hyperadrenocorticism is suspected, perform ACTH stimulation and low-dose dexamethasone suppression tests, imaging,
and other diagnostic tests as needed.
Table 1: Differential Diagnoses in Dogs with Polyuria and Polydipsia
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If these test results are inconclusive or within the reference range, the remaining differential diagnoses include central
diabetes insipidus, psychogenic polydipsia, nephrogenic diabetes insipidus, and early-stage chronic kidney disease. A congenital
form of nephrogenic diabetes insipidus exists, but it is rare and manifests in puppies. Most causes of acquired nephrogenic
diabetes insipidus are among the listed differential diagnoses in Table 1 and need to be definitively diagnosed.
A water deprivation test can be used to differentiate central diabetes insipidus from psychogenic polydipsia; however, this
test involves intentionally dehydrating a patient. Ideally, the glomerular filtration rate should be evaluated before water
deprivation testing is considered in a polyuric, nonazotemic patient for which the remaining differential diagnoses are central
or nephrogenic diabetes insipidus, psychogenic polydipsia, and early-stage chronic kidney disease. But evaluating the glomerular
filtration rate is not usually practical for general practitioners. To avoid the risks associated with a water deprivation
test in an animal that potentially has an abnormal glomerular filtration rate, consider conducting a desmopressin trial, a
safer method for diagnosing central diabetes insipidus.
To conduct this trial, the owner measures the patient's free-choice 24-hour water intake for two or three days and collects
and labels daily urine samples. The specific gravity of the urine samples should be checked. The owner then administers 0.05
to 0.2 mg desmopressin orally every eight hours or 1 to 4 drops of the 100 μg/ml desmopressin nasal spray in one conjunctival
sac every 12 hours for five to seven days. The owner monitors the dog's water consumption and collects and labels urine samples
during the treatment period. Dogs with central diabetes insipidus should respond well to this treatment, forming more concentrated
urine and drinking much less water than they were previously drinking. A moderate response may occur in dogs with partial
central diabetes insipidus or with hyperadrenocorticism.
Even with a desmopressin trial, caution should be exercised in geriatric patients and those having isosthenuria since these
patients are more likely to have early-stage chronic kidney disease, and excessive dehydration could be damaging. Further, if an animal with unexplained polyuria and polydipsia presents to the hospital in a dehydrated state and its urine
specific gravity is < 1.030, a water deprivation test is unnecessary and is contraindicated.
SUGGESTED READING
1. Feldman EC, Nelson RW. Water metabolism and diabetes insipidus. In: Canine and feline endocrinology and reproduction. 3rd ed. St Louis, Mo: Saunders, 2004;2-44.
Marjorie L. Chandler, DVM, MS, MACVSc, DACVN, DACVIM, DECVIM, MRCVS
Hospital for Small Animals
University of Edinburgh
Roslin, Midlothian, Scotland
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