The Disease That Looks Like Aging — Until It Isn’t
Your dog is drinking more water than usual, panting at rest, and developing a pot belly despite eating the same food. The coat is thinning. The energy is fading. Easy to chalk up to getting older. But these are classic signs of Cushing’s disease — and the average delay from first symptoms to diagnosis is 6-12 months, during which metabolic damage accumulates silently.
Cushing’s disease (hyperadrenocorticism, HAC) results from chronic excess cortisol production. In 80-85% of cases, a pituitary adenoma overproduces ACTH, driving bilateral adrenal hyperplasia and excess cortisol secretion (pituitary-dependent, PDH). The remaining 15-20% involve adrenal tumors (adrenal-dependent, ADH) that autonomously produce cortisol.
The signs develop gradually over months: increased thirst and urination, ravenous appetite, pot-bellied appearance from hepatomegaly and abdominal muscle weakness, bilateral symmetric hair loss, thin skin, calcinosis cutis, muscle wasting, panting, and lethargy. Because they overlap so heavily with normal aging, many owners — and some veterinarians — miss the diagnosis until significant metabolic and cardiovascular damage has occurred.
Breed Predisposition
Fracassi et al. (2015) documented breed distribution in naturally occurring PDH:
- Dachshunds: Among the most predisposed breeds
- Miniature Poodles and Toy Poodles: Significantly overrepresented
- Beagles, Boxers, Boston Terriers: Elevated risk
- Yorkshire Terriers, Bichon Frises: Also predisposed
Most dogs are diagnosed between 7 and 12 years of age. Small breeds are overrepresented in pituitary-dependent disease, while larger breeds are more commonly affected by adrenal tumors.
Diagnostic Challenge
Diagnosing Cushing’s disease is notoriously difficult — the ACVIM consensus statement (Behrend et al., 2013) acknowledges that no single test is both highly sensitive and highly specific. The diagnostic approach typically involves:
Screening Tests
- Low-dose dexamethasone suppression test (LDDS): The most widely recommended screening test. Sensitivity approximately 85-95% for PDH but specificity is lower (non-adrenal illness can cause false positives). A normal result effectively rules out HAC.
- ACTH stimulation test: Lower sensitivity (60-85%) but higher specificity. Also used for monitoring trilostane therapy.
- Urine cortisol:creatinine ratio (UCCR): High sensitivity (nearly 100%) but poor specificity. Best used as a screening test — a normal UCCR essentially rules out Cushing’s disease.
Differentiating Tests
Once excess cortisol is confirmed, differentiating pituitary from adrenal origin guides treatment:
- High-dose dexamethasone suppression test: Cortisol suppression suggests pituitary origin; failure to suppress suggests adrenal tumor
- Endogenous ACTH measurement: Elevated ACTH supports pituitary origin; suppressed ACTH supports adrenal origin
- Abdominal ultrasound: Bilaterally enlarged adrenals suggest pituitary-dependent disease; a unilateral adrenal mass with contralateral adrenal atrophy suggests adrenal tumor
The diagnostic workup must be performed carefully because treating a dog that does not have Cushing’s disease — particularly with mitotane — carries serious iatrogenic risk. The ACVIM consensus emphasizes that testing should only be pursued in dogs with compatible clinical signs, not as screening in asymptomatic animals.
Treatment Options
Trilostane (Vetoryl)
Trilostane is a competitive inhibitor of 3-beta-hydroxysteroid dehydrogenase, blocking cortisol synthesis in the adrenal cortex. It has become the first-line medical treatment for both pituitary-dependent and adrenal-dependent HAC in most countries.
Ramsey (2010) reviewed the trilostane evidence:
- Response rate: 70-80% of dogs show significant improvement in clinical signs within 2-4 weeks
- Starting dose: 1-2 mg/kg once daily (some dogs require twice-daily dosing)
- Monitoring: ACTH stimulation test 4-6 hours after dosing at 2 weeks, 4 weeks, 8 weeks, then every 3-4 months
- Side effects: Lethargy, decreased appetite, vomiting, and diarrhea (usually transient). The most serious risk is iatrogenic hypoadrenocorticism (Addisonian crisis) from excessive cortisol suppression, which occurs in 2-5% of dogs and can be fatal if not recognized and treated promptly.
Mitotane (Lysodren)
Mitotane causes selective necrosis of the adrenal cortex, reducing cortisol-producing capacity. It was the standard treatment before trilostane became available.
Barker et al. (2005) compared survival in dogs treated with mitotane versus trilostane and found comparable median survival times (approximately 2 years for both), though mitotane required more intensive monitoring during the induction phase and carried higher risk of serious adverse effects.
Mitotane use has declined in favor of trilostane due to:
- Higher incidence of serious side effects during induction (Addisonian crisis, anorexia, vomiting, diarrhea)
- More complex dosing protocol (induction phase with daily dosing, then maintenance)
- Less predictable dose-response relationship
Mitotane remains an option for dogs that do not respond to trilostane or for adrenal tumors.
Surgery
Adrenalectomy is the definitive treatment for adrenal-dependent HAC. Surgical removal of the cortisol-producing adrenal tumor cures the disease if the tumor is completely excised without metastasis. However, adrenalectomy is technically demanding, carries significant perioperative risk (hemorrhage, thromboembolism), and requires an experienced surgeon. Perioperative mortality ranges from 5-20% depending on tumor size and invasiveness.
Transsphenoidal hypophysectomy (surgical removal of the pituitary tumor) is performed at a small number of referral centers worldwide and offers the possibility of cure for pituitary-dependent disease. Success rates of 80-85% have been reported, but the procedure requires lifelong hormone supplementation (thyroid hormone, cortisol replacement) and is available at very few institutions.
Longevity Impact
Untreated Cushing’s disease significantly shortens lifespan through multiple mechanisms:
- Hypertension: 50-80% of dogs with HAC develop systemic hypertension, which damages the kidneys, eyes, brain, and cardiovascular system
- Thromboembolism: Cortisol excess creates a hypercoagulable state. Pulmonary thromboembolism is a recognized cause of sudden death in Cushingoid dogs.
- Diabetes mellitus: Chronic cortisol-driven insulin resistance may progress to overt diabetes
- Urinary tract infections: Immunosuppression and dilute urine increase UTI frequency
- Skin infections: Cortisol-mediated immune suppression predisposes to pyoderma and demodicosis
- Muscle wasting and weakness: Progressive sarcopenia reduces mobility and quality of life
With appropriate treatment, median survival times are approximately 2 to 2.5 years from diagnosis — limited more by the advanced age at diagnosis and concurrent age-related diseases than by Cushing’s disease itself. Dogs diagnosed earlier in the disease course and managed with consistent monitoring tend to have longer survival times.
Monitoring Protocol
Effective Cushing’s management requires lifelong veterinary monitoring:
- ACTH stimulation tests every 3-4 months (for trilostane-treated dogs)
- Electrolyte panels to detect iatrogenic hypoadrenocorticism
- Blood pressure monitoring every 3-6 months
- Urinalysis and urine culture every 3-6 months (screening for subclinical UTI)
- Clinical reassessment of water intake, appetite, energy, skin and coat condition, and muscle mass
Limitations
The imperfect sensitivity and specificity of Cushing’s diagnostic tests mean that some dogs are treated for a disease they do not have, while others with early disease go undiagnosed. The optimal trilostane dose varies widely between individuals, and the ACTH stimulation test used for monitoring has limitations in predicting clinical response. Long-term outcome data comparing treatment modalities is limited to retrospective studies, and prospective randomized trials comparing trilostane to mitotane have not been conducted.
Frequently Asked Questions
How is Cushing’s disease diagnosed in dogs?
Diagnosis requires a combination of clinical signs (increased drinking, urination, appetite, hair loss, pot-bellied appearance), screening tests (urine cortisol:creatinine ratio, low-dose dexamethasone suppression test, ACTH stimulation test), and imaging to differentiate pituitary-dependent from adrenal-dependent forms. No single test is definitive, and false positives are common in stressed or ill dogs.
What is the life expectancy of a dog with Cushing’s disease?
With appropriate treatment and monitoring, many dogs with pituitary-dependent Cushing’s disease live 2-3 years or more after diagnosis. Prognosis depends on the underlying cause, response to treatment, and presence of complications like diabetes or hypertension. Adrenal tumors have more variable outcomes depending on whether they are benign or malignant.
Is Cushing’s disease treatment expensive?
Yes. Ongoing treatment with trilostane or mitotane requires regular ACTH stimulation tests every 3-6 months to monitor cortisol levels and adjust dosing. Monthly medication costs typically range from $50-200, and monitoring tests add $200-400 per visit. However, untreated Cushing’s disease leads to progressive organ damage and significantly reduced quality of life.
Which breeds are most prone to Cushing’s disease?
Pituitary-dependent Cushing’s disease is most common in small to medium breeds including Poodles, Dachshunds, Boston Terriers, Beagles, and Yorkshire Terriers. It typically develops in middle-aged to older dogs (8-12 years). Larger breeds are more likely to develop adrenal-dependent forms.
Bottom Line
Cushing’s disease mimics normal aging so closely that the average diagnosis is delayed 6-12 months, during which metabolic and cardiovascular damage accumulates. Trilostane is the current first-line treatment, producing clinical improvement in 70-80% of dogs, but requires lifelong monitoring and carries a small risk of Addisonian crisis. With consistent management, median survival is approximately 2-2.5 years from diagnosis — limited more by age at diagnosis and concurrent conditions than by Cushing’s itself.
References
- Behrend EN et al. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM consensus update (Journal of Veterinary Internal Medicine, 2013).
- Ramsey IK. Trilostane in dogs (Veterinary Clinics of North America: Small Animal Practice, 2010).
- Barker EN et al. A comparison of the survival times of dogs treated with mitotane or trilostane for pituitary-dependent hyperadrenocorticism (Journal of Veterinary Internal Medicine, 2005).
- Fracassi F et al. Breed distribution of naturally occurring pituitary-dependent hyperadrenocorticism in dogs (Journal of Veterinary Internal Medicine, 2015).