The Diagnostic Approach to Polyuria in the Dog

Introduction
Urine osmolality (uosm: 161 - 2830) and urine specific gravity (usg: 1.006 - >1.050) vary widely among healthy pet dogs. in some individual dogs uosm fluctuates considerably during the day and uosms close to plasma osmolality (posm) may be reached. in some pet dogs the low uosms are associated with sufficiently high uosms at other times of the day so that the owners do not perceive their dog to be polydipsic or polyuric. However, in other dogs the situation is more pronounced and the animals are presented to the veterinarian because of polyuria and polydipsia. Some of these animals may thus be recognised as having primary polydipsia.

Differential diagnosis
Apart from central diabetes insipidus there are in principal only two basic disorders which can account for increased water diuresis. these disorders are primary polydipsia and nephrogenic diabetes insipidus. primary polydipsia is said to occur in hyperactive young dogs that are left alone during the day for many hours or have gone through significant changes in their environment. it has been observed that placing the animal in a completely different environment may stop the problem. the finding of the above mentioned spontaneous fluctuation of uosm may be regarded as diagnostic of primary polydipsia. However, these irregular patterns of water intake may also be associated with abnormalities in vasopressin release. there are a few individual case reports of congenital nephrogenic diabetes insipidus, the condition in which the kidney tubules are insensitive/less sensitive to the action of antidiuretic hormone.

In addition to these two basic and infrequently encountered differential diagnoses, a wide variety of conditions causes polyuria. in the young animal this may be congenital kidney disease, whereas at all ages acquired kidney disease may lead to polyuria. Especially in the middle-aged and elderly animals (endocrine) conditions such as hyper- adrenocorticism, hyperthyroidism, pyometra, progestin-induced (luteal phase) growth- hormone excess, hyperparathyroidism and hypercalcemia of malignancy have to be considered. in several of these conditions impaired release of vasopressin and/or interference with its action may play a role in the polyuria. [...]