Diagnostic Options for Cushing's Disease

Hypercortisolism is a common condition in dogs and may be iatrogenic or spontaneous. Up to 85% of spontaneous cases are adrenocorticotropic hormone (ACTH)–dependent, usually due to a pituitary corticotroph adenoma. Diagnosis depends on demonstrating increased production of cortisol or decreased sensitivity to glucocorticoid feedback. This paper discusses several methods of diagnosing hypercortisolism; measurement of urinary corticoids is replacing the low-dose dexamethasone suppression test (LDDST) because it takes less time and is noninvasive. Because urine is stored and mixed in the bladder for several hours, the cortisol levels adjust for fluctuations that may be found in plasma concentrations. Urinary corticoids are related to the urine creatinine that creates the urinary corticoid:creatinine ratio (UCCR). Dogs with few clinical signs but with high UCCRs should be evaluated for nonadrenal disease that may be causing endogenous stress and increased cortisol levels. When there is still a suspicion of hypercortisolism, but with inconclusive UCCR or IV-LDDST results, an oral LDDST may be performed.

Commentary: The urine cortisol:creatinine ratio is a very sensitive diagnostic test for diagnosing Cushing’s disease; however, it has poor specificity. As a result, it is commonly used to try to remove Cushing’s from the differential list in a patient with clinical signs or biochemical changes that could be due to hypercortisolism, as a normal result makes a diagnosis of Cushing’s unlikely. An oral dexamethasone suppression test can be useful in patients with equivocal results on more traditional diagnostics; however, owner compliance must be ensured when performing this diagnostic.

Recent advances in the diagnosis of Cushing’s syndrome in dogs. Kooistra HS, Galac S. VET CLIN NORTH AM SMALL ANIM PRACT 40:249-267, 2010.