Trilostane is a currently approved treatment for canine hyperadrenocorticism in Europe, Australia, New Zealand, and the United States. Adrenocorticotropic hormone (ACTH) stimulation tests should be performed 10 to 14 days after initiating therapy or dose adjustment; an additional ACTH stimulation test should be performed at 28 days and repeated every 3 months thereafter. Testing is performed to minimize the risk for iatrogenic hypocortisolemia and ensure adequate control of hyperadrenocorticism. Each ACTH stimulation test requires at least 60 minutes of hospital time and substantial financial and logistic commitments by the pet owner. The relationship between baseline and post-ACTH stimulation cortisol concentrations was investigated in 103 dogs treated with trilostane. A significant correlation was identified between the 2 concentrations. A range for the baseline cortisol concentrations that reliably indicated acceptable control of adrenal gland function was also defined. A patient with normal thirst, urination, appetite, and physical examination findings and baseline cortisol concentrations between 1.3 mcg/dL and either 2.9 mcg/dL or <_ 50% of the pretreatment baseline cortisol concentration may safely continue to receive current trilostane doses. In addition, baseline cortisol ≥ 1.3 mcg/dL can reliably exclude trilostane overdose in a sick patient or during patient reevaluation following a reduction in trilostane dose.

Commentary: Measurement of baseline cortisol concentrations is not intended to replace ACTH stimulation testing when a more detailed adrenal gland evaluation is required. The limitations of baseline cortisol concentrations in monitoring treatment should be carefully weighed by both pet owners and clinicians; monitoring baseline cortisol concentrations should be used only in patients where cost is an issue for the owner. However, measurement of baseline cortisol levels in dogs with Cushing’s disease undoubtedly provides a simpler and more affordable alternative for owners to provide care for their dogs and will potentially improve owner compliance in treating the disease.—Heather Troyer, DVM, Diplomate ABVP (Canine & Feline Practice)

Evaluation of the use of baseline cortisol concentration as a monitoring tool for dogs receiving trilostane as a treatment for hyperadrenocorticism. Cook AK, Bond AG. JAVMA 237:801-805, 2010.