Canine Polyuria/Polydipsia: When Is Testing for Hyperadrenocorticism Indicated?
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Abby, a 9-year-old female spayed miniature poodle, was presented for polyuria/polydipsia (PU/PD) of several weeks’ duration. Abby had been drinking more, asking to go out more frequently, and urinating larger volumes, which had the owner concerned about a possible UTI. Abby was up to date on vaccines (ie, rabies, Bordetella bronchiseptica, leptospirosis, distemper/adenovirus/ parainfluenza/parvovirus) and parasite prevention. Her medical history was unremarkable, except for a history of medial patellar luxation. She had not received any medications except parasite prevention in the preceding months.
On examination, Abby weighed 18 lb (8.2 kg) and had a BCS of 4/5. Mucous membranes were pink and moist, and capillary refill time was <2 seconds. Thoracic auscultation was unremarkable, and pulses were strong and synchronous. She had mild dental calculus and a slightly distended, pendulous abdomen, with no palpable fluid wave. Aside from the previously noted medial patellar luxation, no other abnormalities were observed.
CBC, serum chemistry, total thyroxine (T4), and urinalysis were recommended based on examination findings and the presence of PU/PD. Abby’s owner expressed financial constraints but authorized the urinalysis, saying she would consider the cost of further testing.
A urine sample was collected via cystocentesis. Urinalysis revealed a urine specific gravity of 1.006 and an inactive sediment. Culture was performed to rule out occult infection and was negative.
After receiving these results, Abby’s owner authorized the additional recommended tests. One week later, Abby returned for fasting bloodwork. CBC was unremarkable, and T4 was within normal limits. Serum chemistry profile revealed increased ALP, ALT, and cholesterol.
Differentials for Canine Polyuria/Polydipsia
Considering Abby’s blood work and urinalysis results, her veterinarian began to work through the long list of differentials for PU/PD.1 Based on Abby’s laboratory results, examination findings, and medical history, many of the differentials could be ruled out, leaving the following as remaining differentials:
Hyperadrenocorticism
Atypical Cushing’s disease (hyperaldosteronism)
Hypoadrenocorticism
Liver disease
Neoplasia
Central diabetes insipidus
Primary nephrogenic diabetes insipidus
Renal medullary washout
Acute kidney injury
Pheochromocytoma
Psychogenic polydipsia
After reviewing this list, Abby’s veterinarian strongly suspected hyperadrenocorticism. Her signalment was appropriate (ie, a middle-aged miniature poodle) and she also had characteristic laboratory (ie, increased ALP/ALT and hypercholesterolemia) and examination findings (ie, pendulous abdomen).
Diagnostic Tests for Cushing's Disease
There are several tests that can be used to diagnose hyperadrenocorticism, each with their own unique advantages and disadvantages. The urine cortisol:creatinine ratio can be used to help rule out Cushing’s disease when there is a low index of suspicion. A negative result effectively removes hyperadrenocorticism from the differential list; however, false-positive results are relatively common, especially in dogs with nonadrenal disease, so a positive result is not generally helpful.2 Because Abby’s veterinarian strongly suspected Cushing’s disease, this test was not a good option for Abby.
The low-dose dexamethasone suppression (LDDS) test is commonly used to diagnose Cushing’s disease. It is relatively inexpensive as compared with the other common diagnostic test, the ACTH stimulation test. In addition, the LDDS test can serve as a differentiating test, distinguishing between adrenal-dependent and pituitary-dependent disease in ≤60% of cases.3 If a nonadrenal illness is present, however, the LDDS test may produce a false-positive result.3
The LDDS test can serve as a differentiating test, distinguishing between adrenal-dependent and pituitary-dependent disease in ≤60% of cases.
The ACTH stimulation test is another common diagnostic test for Cushing’s disease. It is more effective in diagnosing iatrogenic hyperadrenocorticism than the LDDS test and has a higher specificity.1 However, it cannot differentiate pituitary-dependent from adrenal-dependent Cushing’s disease, thus requiring follow-up testing for a positive result.
Abby’s veterinarian elected to perform the LDDS test. This was a good option for Abby because of the test’s relatively low cost, lack of evidence of nonadrenal illness, and the fact that it can serve as a differentiating test.
Abby was brought in for an all-day visit to the clinic. The veterinary team collected a baseline blood sample, administered dexamethasone (0.01-0.015 mg/kg IV), and collected blood samples 4 and 8 hours after dexamethasone injection. Abby’s cortisol levels were as follows:
Before dexamethasone injection: 10 ug/dL
4 hours postinjection: 4 ug/dL
8 hours postinjection: 10 ug/dL
These results show suppression of cortisol production at 4 hours, followed by an escape of suppression at 8 hours. This pattern is diagnostic for pituitary-dependent Cushing’s disease. Based on these results, Abby’s veterinarian contacted the owner to recommend trilostane treatment.
Conclusion
When middle-aged dogs are presented for PU/PD, the top 3 differentials should generally be hyperadrenocorticism, diabetes mellitus, and chronic renal disease. However, it is important to also think through other differential diagnoses for PU/PD. Some of these differentials can be ruled out based on examination/laboratory findings and history, whereas others require more advanced testing. The diagnostic investigation of PU/PD requires veterinarian discretion in determining when to look for Cushing’s disease with specific tests such as the LDDS test or begin investigation for other differentials.
By thinking through this case critically and considering other differentials for Abby’s PU/PD, Abby’s veterinarian avoided LDDS testing until there was a high clinical index of suspicion. This not only minimized the likelihood of stressing Abby and her owner with unnecessary tests, but it also ensured responsible use of the owner’s available financial resources.
