The physical examination findings of dogs with diabetes insipidus are generally normal or unremarkable. The results of routine blood work (a complete blood count and serum biochemistry profile) may or may not be helpful. However, a urinalysis will usually show abnormally dilute, or unconcentrated, urine. In medical lingo, this is referred to as low urine specific gravity, or “isosthenuria”. Culturing a urine sample, conducting a blood test for Cushing’s disease (hyperadrenocorticism) and conducting abdominal ultrasound, may all be helpful to rule in or out a number of causes of nephrogenic diabetes insipidus.
Advanced diagnostic tools and procedures can help differentiate between central or nephrogenic diabetes insipidus on the one hand (CDI or NDI), and psychogenic or idiopathic polydipsia on the other. These may include assessment of the dog’s clearance of creatinine, assessment of something called plasma osmolality and a procedure called nuclear scintigraphy. A modified water deprivation test can be performed, but it must be done extremely carefully, under strict veterinary supervision and only on dogs that are properly hydrated and do not have kidney disease. The water deprivation test requires hourly monitoring that is rigidly adhered to, and should be done in a veterinary hospital. Computed tomography (CT scan) and magnetic resonance imaging (MRI) can be used to identify masses or other physical abnormalities in the pituitary gland and hypothalamus. All of these advanced diagnostic techniques should be discussed in much greater detail with a veterinarian.
Sometimes, administering a synthetic version of antidiuretic hormone (ADH), which is called desmopressin acetate, or DDAVP, in a therapeutic ADH supplementation trial, is a less risky way to differentiate between CDI, NDI and psychogenic polydipsia than is performing a water deprivation test. Dogs with CDI will respond to empirical treatment with DDAVP, while those with NDI or psychogenic polydipsia will not.