Trilostane for Dogs: How It Works, Dosing, and Cushing's Management
Trilostane for dogs treats Cushing's disease by blocking cortisol production. Learn the science, dosing, side effects, costs, and what owners need to know.
Direct Answer: Trilostane (brand name Vetoryl) is a prescription medication used to treat hyperadrenocorticism (Cushing's disease) in dogs. It works by inhibiting the enzyme 3-beta-hydroxysteroid dehydrogenase (3β-HSD), which blocks the adrenal glands' ability to produce cortisol. Most dogs show clinical improvement within 2 to 4 weeks of starting treatment. Trilostane is FDA-approved for canine use and is considered the first-line medical treatment for pituitary-dependent Cushing's disease, which accounts for approximately 80–85% of all canine Cushing's cases. Typical costs range from $40 to $120 per month depending on your dog's weight and dosage.
The Endocrinology Behind Cushing's Disease in Dogs
To understand why trilostane works, you need to understand what goes wrong in Cushing's disease. The hypothalamic-pituitary-adrenal (HPA) axis is a feedback loop: the hypothalamus releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to release adrenocorticotropic hormone (ACTH), which in turn tells the adrenal glands to produce cortisol. In a healthy dog, rising cortisol levels signal the pituitary to reduce ACTH output — a negative feedback loop.
In pituitary-dependent hyperadrenocorticism (PDH), a small benign tumor (adenoma) on the pituitary gland pumps out ACTH continuously, ignoring the feedback signal. The adrenal glands, constantly stimulated, hypertrophy and overproduce cortisol. In adrenal-dependent Cushing's (ADH), which accounts for roughly 15–20% of cases, the tumor sits directly on one adrenal gland, causing unilateral cortisol overproduction independent of ACTH.
How Trilostane Intervenes in the HPA Axis
Trilostane is a synthetic steroid analog that competitively inhibits the enzyme 3β-hydroxysteroid dehydrogenase in the adrenal cortex. This enzyme is required to convert pregnenolone into progesterone — an early, critical step in the steroidogenesis pathway. By blocking this conversion, trilostane effectively reduces the adrenal glands' capacity to synthesize cortisol, aldosterone, and other steroid intermediates.
Importantly, trilostane does not shrink pituitary tumors or address the root cause of PDH. It manages cortisol output at the adrenal level. The pituitary tumor continues to produce ACTH, which is why ACTH levels in treated PDH dogs often rise further — the pituitary senses low cortisol and increases signaling. This is expected and monitored through ACTH stimulation testing.
Evolutionary Context: Why Dogs Are Vulnerable to Cushing's
Ancestral cortisol regulation: In wild canids — wolves, coyotes, and early domesticated dogs — the HPA axis evolved to handle acute stressors: a hunt, a territorial confrontation, a predator encounter. Cortisol spikes were brief and functional, mobilizing glucose for fight-or-flight responses before returning to baseline. Chronic, unregulated cortisol elevation simply did not occur in natural environments because the negative feedback loop remained intact.
Modern domestic dogs live longer (many breeds now average 12–15 years versus 6–8 years for wild wolves) and face fewer acute survival stressors but more chronic ones: environmental allergens, sedentary lifestyles, and extended lifespans that allow slow-growing pituitary adenomas to develop and cause clinical disease. Cushing's disease is overwhelmingly a condition of middle-aged to older dogs (average onset 7–12 years), reflecting the evolutionary mismatch between extended domestic lifespans and tumor surveillance mechanisms that evolved for shorter-lived animals.
Breed predisposition data supports a genetic component. Poodles (especially miniature and toy), Dachshunds, Boston Terriers, Yorkshire Terriers, and Beagles show statistically higher incidence rates. A study published in the Journal of Veterinary Internal Medicine found that dogs weighing less than 20 kg (44 lb) had a significantly higher risk of PDH, suggesting body size and genetic lineage interact with tumor development.
Trilostane Dosing, Administration, and Monitoring Protocol
Trilostane is dosed based on body weight and administered orally, typically once daily with food. Food significantly improves absorption — studies show bioavailability increases by approximately 50% when given with a meal. The starting dose recommended by the manufacturer (Dechra Veterinary Products) and supported by the Merck Veterinary Manual is as follows:
| Dog Weight | Starting Dose | Capsule Size | Estimated Monthly Cost |
|---|---|---|---|
| 5–10 lb (2.3–4.5 kg) | 10 mg | 10 mg capsule | $40–$55 |
| 10–20 lb (4.5–9 kg) | 30 mg | 30 mg capsule | $55–$75 |
| 20–40 lb (9–18 kg) | 60 mg | 60 mg capsule | $70–$90 |
| 40–80 lb (18–36 kg) | 120 mg | 120 mg capsule | $85–$110 |
| 80+ lb (36+ kg) | 120–240 mg | 120 mg capsules | $100–$150 |
Monitoring schedule: Your veterinarian will typically run an ACTH stimulation test at 10–14 days after starting trilostane, again at 30 days, then at 90 days, and every 3–6 months thereafter. Each ACTH stim test costs approximately $150–$300 depending on your region and clinic. Blood work (complete blood count, chemistry panel, electrolytes) should accompany each monitoring visit to screen for Addisonian crisis — the condition of over-suppression where cortisol drops too low.
Dose adjustments are common. In a multi-center clinical trial supporting FDA approval, approximately 82% of dogs required at least one dose adjustment during the first year. Some dogs need twice-daily dosing if once-daily administration fails to control cortisol for the full 24 hours. Your vet will determine this based on pre-pill cortisol levels measured at the trough (just before the next dose is due).
Emergency Red Flags — Contact Your Vet Immediately
- Severe lethargy or collapse — may indicate Addisonian crisis (cortisol too low)
- Vomiting and diarrhea simultaneously — suggests adrenal insufficiency
- Loss of appetite lasting more than 24 hours
- Weakness in hind legs or inability to stand
- Dark or tarry stools — possible gastrointestinal ulceration
If you suspect trilostane overdose or Addisonian crisis, bring your dog to an emergency clinic immediately. Treatment involves IV fluids and glucocorticoid supplementation. You can also contact the ASPCA Animal Poison Control Center at (888) 426-4435 (24/7, consultation fee applies).
Clinical Outcomes: What Research Shows
The pivotal clinical study for Vetoryl (trilostane) demonstrated that approximately 80–85% of treated dogs showed significant improvement in clinical signs within 4 to 12 weeks. The most commonly reported improvements, in order of how quickly owners notice them:
- Reduced water consumption and urination — often within 2–4 weeks (polydipsia and polyuria are typically the first signs to resolve)
- Improved appetite regulation — ravenous hunger decreases within 3–6 weeks
- Reduced panting — noticeable within 4–8 weeks as abdominal muscle tone improves
- Coat regrowth — hair regrowth is the slowest improvement, often taking 3–6 months, and may be incomplete in some dogs
- Reduced pot-bellied appearance — abdominal muscle strengthening occurs over 2–4 months
Median survival time for dogs with PDH treated with trilostane is approximately 2 to 2.5 years from diagnosis, according to a retrospective study in the Journal of the American Veterinary Medical Association. However, many dogs live considerably longer — 3 to 5 years — particularly when the pituitary tumor remains small (microadenoma). Dogs with macroadenomas (tumors larger than 1 cm) may have shorter survival times due to neurological compression, and these cases often require additional intervention such as radiation therapy.
Common Misconceptions About Trilostane and Cushing's
| Myth | Fact |
|---|---|
| "Trilostane cures Cushing's disease." | Trilostane manages cortisol levels but does not eliminate the underlying pituitary or adrenal tumor. It is a lifelong treatment. If discontinued, clinical signs return within days to weeks. |
| "My dog's symptoms are just old age, not a disease." | Increased thirst, urination, hair loss, and a pot-bellied appearance are NOT normal aging. These are clinical signs of hyperadrenocorticism and warrant diagnostic testing. |
| "Once my dog looks better, I can stop the medication." | Stopping trilostane without veterinary guidance risks cortisol rebound. The pituitary tumor is still producing ACTH, and abrupt withdrawal can cause adrenal crisis or rapid symptom recurrence. |
| "Trilostane is the same as human Cushing's medication." | Human Cushing's is often treated with ketoconazole, metyrapone, or surgery. Trilostane is specifically formulated and FDA-approved for dogs. Human formulations and dosages are not interchangeable. |
| "All dogs with Cushing's need trilostane." | Some dogs with mild, non-progressive Cushing's (especially atypical cases) may be managed with monitoring alone. Adrenal-dependent Cushing's may be better treated with surgical adrenalectomy. Your veterinarian determines the best approach. |
Trilostane vs. Mitotane: How Do They Differ?
Before trilostane received FDA approval for veterinary use in 2008, mitotane (Lysodren) was the standard treatment. Mitotane works by selectively destroying the zona fasciculata and zona reticularis layers of the adrenal cortex — essentially chemically ablating the tissue that produces cortisol. This is a fundamentally different mechanism from trilostane's reversible enzyme inhibition.
| Feature | Trilostane (Vetoryl) | Mitotane (Lysodren) |
|---|---|---|
| Mechanism | Reversible enzyme inhibition (3β-HSD) | Adrenocortical tissue destruction |
| Reversibility | Yes — effects reverse if medication is stopped | No — damage is permanent; risk of permanent Addison's disease |
| FDA Status | FDA-approved for dogs | Used off-label (human drug) |
| Safety Profile | Generally well-tolerated; lower risk of Addisonian crisis | Higher risk of adrenal necrosis and permanent hypoadrenocorticism |
| Cost (monthly) | $40–$150 | $30–$80 |
| Loading Phase | No — start at target dose and adjust | Yes — aggressive loading phase followed by maintenance |
Most veterinary endocrinologists now recommend trilostane as the first-line treatment due to its reversibility and wider safety margin. Mitotane may still be considered in cases where trilostane is ineffective, cost-prohibitive, or when the owner prefers the possibility of a permanent (but controlled) resolution through adrenal ablation.
Practical Daily Management for Owners
What Life on Trilostane Looks Like Day-to-Day
- Administer consistently with food — the same meal each day (morning or evening, but be consistent). Splitting the capsule is not recommended because the contents are not uniformly distributed.
- Track water intake — measure your dog's daily water consumption. A return to excessive drinking (more than 100 mL/kg/day) may signal that the dose needs adjustment.
- Keep a symptom journal — note changes in appetite, coat quality, energy levels, and urination frequency. Bring this to every monitoring appointment.
- Never double-dose — if you miss a dose, give it when remembered if it's within a few hours; otherwise skip and resume the next day. A single missed dose will not cause a clinical crisis.
- Store at room temperature (below 77°F / 25°C) in the original container. Do not refrigerate. Capsules are moisture-sensitive.
- Budget for monitoring — expect to spend $600–$1,200 annually on ACTH stim tests and blood work, in addition to medication costs. Total annual management cost for a medium-sized dog typically ranges from $1,200 to $2,500.
When Cushing's Signs Might Signal Something Else
Not every dog with increased thirst, hair loss, and a pot-bellied appearance has Cushing's disease. Several conditions share overlapping clinical signs and must be ruled out through diagnostic testing:
- Diabetes mellitus — causes polydipsia and polyuria but is differentiated by persistent hyperglycemia and glucosuria on urinalysis
- Hypothyroidism — can cause hair loss and lethargy but typically presents with weight gain and cold intolerance rather than increased appetite
- Chronic kidney disease — causes increased thirst and urination but is identified through elevated BUN, creatinine, and SDMA on blood panels
- Liver disease — can cause elevated ALP (alkaline phosphatase), which is also commonly elevated in Cushing's, making differentiation challenging without further testing
- Atypical Cushing's — some dogs show clinical signs with normal cortisol levels but elevated sex hormones (progesterone, androstenedione, estradiol) produced by the adrenal glands. Standard ACTH stim tests may miss this; a sex hormone panel is required
The gold-standard diagnostic protocol involves a combination of screening tests: the low-dose dexamethasone suppression test (LDDST), the ACTH stimulation test, and the urine cortisol:creatinine ratio (UC:Cr). Imaging (abdominal ultrasound) helps differentiate PDH from ADH by visualizing adrenal gland size and symmetry. Diagnostic workup costs typically range from $400 to $800 before treatment begins.
Frequently Asked Questions
Can trilostane be used in cats or other animals?
Trilostane is used off-label in cats with hyperadrenocorticism, though feline Cushing's is rare and often associated with diabetes mellitus. It is also used in ferrets with adrenal disease. Dosing and monitoring protocols differ significantly from canine protocols. Always consult a veterinarian experienced in the species being treated.
Is trilostane safe for long-term use?
Yes. Long-term studies (3+ years) have shown trilostane to be well-tolerated in most dogs when properly monitored. The primary long-term risk is progressive adrenal hyperplasia — the adrenal glands may enlarge as they attempt to overcome the enzyme blockade. Regular ultrasound monitoring helps detect this. Some dogs on long-term trilostane develop mild electrolyte imbalances (elevated potassium) that require dose adjustment.
Can I give trilostane with other medications?
Trilostane has relatively few drug interactions, but caution is warranted with ACE inhibitors (e.g., enalapril, benazepril), potassium-sparing diuretics (e.g., spironolactone), and NSAIDs. These combinations can increase the risk of hyperkalemia (elevated potassium). Always provide your veterinarian with a complete medication list, including supplements and over-the-counter products.
What happens if I stop trilostane suddenly?
Because trilostane's effects are reversible, stopping the medication allows cortisol production to resume, typically within 24–48 hours. Clinical signs of Cushing's will return within 1–4 weeks. Sudden discontinuation does not cause an Addisonian crisis the way stopping mitotane might, but the uncontrolled cortisol elevation carries its own risks, including immunosuppression, muscle wasting, and increased susceptibility to infections.
How does Cushing's disease differ in dogs versus humans?
In humans, Cushing's disease is most commonly caused by a pituitary adenoma (similar to canine PDH), but iatrogenic Cushing's — caused by prolonged corticosteroid medication use — is also very common. In dogs, iatrogenic Cushing's occurs but is less prevalent than naturally occurring PDH. Additionally, dogs more frequently present with dermatological signs (bilateral symmetrical alopecia, calcinosis cutis) that are less common in human presentations. Treatment approaches also differ: humans often undergo transsphenoidal pituitary surgery, while dogs are primarily managed medically.
Veterinary Disclaimer: This article is for educational purposes only and does not replace professional veterinary advice. Trilostane is a prescription medication that must be managed by a licensed veterinarian. Dosing, monitoring, and treatment decisions should always be made in consultation with your veterinarian or a board-certified veterinary internist. If your dog is showing signs of Cushing's disease, schedule a diagnostic appointment with your vet before starting any treatment.
Aaron Whyte
All our authors care for dogs every day — read more of their work on the authors page.
