Understanding Cushing’s Disease in Dogs

As our beloved canine companions age, certain changes in their appearance and behavior are often dismissed as simple signs of getting older. They might drink a little more water, seem a bit sluggish, or develop a noticeable “pot belly.” While aging certainly brings changes, sometimes these subtle shifts are red flags signaling a deeply rooted hormonal imbalance known as Cushing’s Disease (medically termed hyperadrenocorticism).

Cushing’s is one of the most common endocrine disorders affecting middle-aged and senior dogs. While a diagnosis can sound daunting, understanding this condition—what causes it, how it manifests, and how it is managed—is the first crucial step toward ensuring your dog maintains the best possible quality of life.

Here, we provide a comprehensive look at Cushing’s Disease, exploring the complex interplay of hormones, the varied symptoms, and the personalized treatment paths available.

Cushing’s Demystified: The Overproduction of Cortisol

At the heart of Cushing’s disease lies a common hormone: cortisol. Often dubbed the body’s “stress hormone,” cortisol is vital for regulating metabolism, controlling blood sugar levels, reducing inflammation, and helping the body respond to stress. It is produced and released by the adrenal glands, two small, often overlooked organs situated near the kidneys.

In a normal, healthy dog, cortisol levels rise and fall in a tightly controlled cycle. However, when a dog develops Cushing’s disease, the adrenal glands begin producing and releasing cortisol in chronic excess. This persistent overexposure to high cortisol is what gradually leads to the cascade of debilitating symptoms.

Imagine a car constantly running its engine at maximum speed: eventually, parts start to break down. Similarly, chronic high cortisol places extreme stress on multiple bodily systems, including the skin, muscles, immune system, and cardiovascular system.

1. The Roots of the Problem: Causes of Hyperadrenocorticism

Cushing’s disease is not a single ailment but rather a syndrome resulting from three distinct underlying causes. Identifying the specific source of the imbalance is paramount, as the cause dictates the necessary treatment approach.

A. Pituitary-Dependent Hyperadrenocorticism (PDH)

This form accounts for approximately 80–85% of all naturally occurring cases of Cushing’s disease.

The pituitary gland, located at the base of the brain, is often called the body’s “master gland.” It produces a hormone called Adrenocorticotropic Hormone (ACTH), which stimulates the adrenal glands to produce cortisol.

In PDH, a usually benign tumor (adenoma) develops on the pituitary gland. This tumor constantly pumps out excessive ACTH, relentlessly driving the adrenal glands to overproduce cortisol. Both adrenal glands become enlarged as they work overtime. Because the tumor is generally microscopic and non-invasive, treatment focuses on controlling the resulting cortisol imbalance rather than removing the brain tumor itself.

B. Adrenal-Dependent Hyperadrenocorticism (ADH)

This form is less common, making up 15–20% of cases. In ADH, the primary issue lies directly within the adrenal glands themselves. A tumor develops on one or both glands, causing it to autonomously produce cortisol, regardless of signals from the pituitary gland.

Adrenal tumors can be either benign (adenoma) or malignant (carcinoma). Because the adrenal gland is overproducing cortisol independently, the pituitary gland often attempts to compensate by shutting down its ACTH release, leading to the atrophy (shrinking) of the unaffected adrenal gland. Diagnosing location is crucial here, as malignant tumors carry a much more guarded prognosis and may require surgical intervention.

C. Iatrogenic Cushing’s Disease

This is the only type of Cushing’s disease that is not naturally occurring. Iatrogenic hyperadrenocorticism is actually caused by external medication—specifically, the prolonged and often high-dose use of corticosteroid medications (such as prednisone, prednisolone, or dexamethasone).

Corticosteroids are powerful drugs used to treat conditions like allergies, inflammation, and autoimmune disorders. While highly effective, using them over a long period mimics the effect of excessive natural cortisol. When a dog is given these steroids, the body stops producing its own cortisol, and these drugs take over. Treatment for iatrogenic Cushing’s always involves slowly and carefully tapering the dosage of the offending medication under strict veterinary supervision. It is critical never to abruptly stop steroid medication, as this can lead to a life-threatening adrenal crisis.

2. Recognizing the Signs: Common Symptoms of Cushing’s

The symptoms of Cushing’s disease can often be confused with normal signs of aging, which is why early and accurate diagnosis is challenging but necessary. Since cortisol influences nearly every tissue type, the visible signs are wide-ranging.

If you observe several of these signs concurrently, it warrants an immediate discussion with your veterinarian.

The Classic ‘P’ Symptoms

• Polyuria (Increased Urination) and Polydipsia (Increased Thirst): This is often the most noticeable symptom. Excess cortisol interferes with the kidney’s ability to concentrate urine, leading to frequent, large volumes of urination. The dog drinks more water to compensate for the fluid loss. Owners often notice increased accidents in the house or excessive soaking of the yard.
• Polyphagia (Increased Appetite): High cortisol levels stimulate the appetite centers in the brain, making the dog seem constantly hungry, sometimes relentlessly searching for food.
• Panting (Excessive and Persistent): Cushing’s is associated with muscle weakness, which affects the respiratory muscles. High cortisol also causes generalized lethargy, and the dog may pant excessively even when resting or in cool environments.
• Pot-Bellied Appearance: This is the hallmark physical sign. The appearance is not solely due to weight gain; rather, it is caused by two factors:
  1. Fat redistribution to the abdomen.
  2. Significant weakening and stretching of the abdominal wall musculature due to cortisol-induced muscle breakdown.

Skin and Coat Changes

Cortisol is destructive to skin health and wound healing:

• Bilateral Hair Loss (Alopecia): Hair loss typically occurs symmetrically on the trunk, flanks, and belly, though the head and legs usually retain hair. The remaining coat may be dull, thin, and brittle.
• Thin, Fragile Skin: The skin becomes very thin and often appears bruised easily.
• Skin Infections: Cortisol suppresses the immune system, leading to recurrent or chronic skin infections (pyoderma), which can manifest as dark pigmentation or persistent scaling.
• Calcinosis Cutis: In advanced cases, hard, chalky deposits of calcium can form in the skin.

General Systemic Symptoms

• Lethargy and Muscle Weakness: Cortisol is catabolic, meaning it breaks down protein. Over time, muscle loss in the limbs is noticeable, leading to difficulty jumping, climbing stairs, or prolonged exercise.
• Weight Gain: Despite muscle loss, fat accumulation leads to overall weight gain.

3. The Diagnostic Journey: Ruling Out the “Great Mimics”

Because many symptoms of Cushing’s—such as increased thirst, hair loss, and lethargy—can be caused by other common geriatric diseases (diabetes, hypothyroidism, kidney failure), Cushing’s is often called the “Great Mimic.” A definitive diagnosis requires a strategic, multi-step process.

Initial Screening Tests

A veterinarian will typically start with routine blood and urine tests:

1. Blood Chemistry Panel: A strong indicator is a significantly elevated level of Alkaline Phosphatase (ALP), an enzyme often released by the liver in response to high cortisol. Blood sugar levels may also be slightly elevated, though usually not to the level seen in diabetes.
2. Urinalysis: A highly dilute urine specific gravity (due to the excessive drinking/urinating) is common.

Specific Endocrine Function Tests

If the screening tests suggest Cushing’s, specific hormonal tests are performed to confirm the diagnosis:

• ACTH Stimulation Test: This test involves taking a baseline blood sample, injecting synthetic ACTH (the hormone that stimulates the adrenal glands), and then taking a second blood sample about an hour later. In a Cushingoid dog, the second sample will show an exaggerated or “hyper-responsive” spike in cortisol production. This test is excellent for confirming Cushing’s and is also essential for monitoring treatment effectiveness.
• Low-Dose Dexamethasone Suppression Test (LDDST): This test checks if the body’s normal feedback loop is working. Dexamethasone (a potent steroid) is given, and blood samples are taken over an extended period (typically 8 hours). In a healthy dog, the dexamethasone should suppress cortisol production. In a dog with Cushing’s, suppression does not occur. This test is often preferred for ruling out Cushing’s, but it requires the dog to stay at the clinic for much of the day.

Imaging Studies

Once the diagnosis is confirmed, imaging helps differentiate between PDH and ADH:

• Abdominal Ultrasound: A highly skilled veterinarian can visualize the adrenal glands. If both glands are enlarged, it strongly suggests PDH. If one gland is enlarged and the other is small (atrophied), it suggests ADH (an adrenal tumor).

4. Tailored Treatment Plans and Management

Cushing’s is usually a chronic condition that is managed, not cured, with the goal being to control cortisol production to alleviate symptoms and prevent secondary complications.

Medical Management (For Pituitary-Dependent Disease)

The vast majority of Cushing’s cases are treated medically:

• Trilostane (Vetoryl): This is the drug most commonly prescribed. Trilostane works by temporarily inhibiting the enzymes necessary for cortisol synthesis in the adrenal glands. It lowers cortisol levels safely, allowing the dog’s body systems to recover.
  • Management Notes: Trilostane therapy is not always straightforward. It requires careful, individualized dosing and frequent follow-up ACTH stimulation tests (initially 10–14 days after starting treatment, then every 3–6 months) to ensure cortisol levels are reduced into the optimal range—neither too high nor too low.

Surgical Intervention (For Adrenal-Dependent Disease)

If the dog has an adrenal tumor (ADH), surgery may be the best option, especially if the tumor is large, appears malignant, or has been successfully staged (meaning it has not spread).

• Adrenalectomy: This involves the highly specialized surgical removal of the affected adrenal gland. This is a complex operation with a significant recovery period, often requiring intensive care afterward, as the dog’s body must adjust to the sudden lack of cortisol. If successful, surgery can be curative.

Radiation Therapy

For large, invasive pituitary tumors (which are very rare but can cause neurological signs), radiation therapy may be considered, although this is typically reserved for specialized veterinary centers.

5. Quality of Life: Living with Cushing’s Disease

A diagnosis of Cushing’s disease is not a death sentence; with diligent care, dogs can enjoy many happy, comfortable years. Management is a team effort between the pet owner and the veterinary team.

Consistent Monitoring is Key

The success of medical management hinges on strict adherence to the treatment schedule and vigilant monitoring at home.

1. Medication Routine: Administering medication precisely as prescribed is non-negotiable. Missing doses or giving too much can have serious consequences.
2. Symptom Tracking: Pet owners must track and report subtle changes. A reduction in excessive thirst and urination is often the first sign that the medication is working. Weight, appetite, and panting frequency should also be monitored.
3. Routine Bloodwork: The regular follow-up ACTH stimulation tests are essential to ensure the dose is effective without being too effective (which could cause Addisonian crisis—too little cortisol).

Supportive Care at Home

While medication addresses the core hormonal imbalance, general care improves comfort:

• Diet: Focus on a high-quality, easily digestible diet. Given the muscle wasting caused by cortisol, many veterinarians recommend a diet with adequate protein levels to help support muscle mass.
• Exercise: Gentle, regular exercise is important to maintain muscle tone and mobility, counteracting the weakness caused by the disease. Avoid strenuous activity during treatment initiation or when the dog feels overly lethargic.
• Skin Care: Due to the risk of recurring skin infections, extra attention should be paid to bathing and hygiene, often requiring medicated shampoos prescribed by your vet.

Conclusion

Cushing’s disease is a serious and pervasive endocrine disorder, but the outlook for most diagnosed dogs is positive once treatment begins. By understanding the three potential causes and recognizing the cluster of symptoms—from the classic pot belly and increased water intake to hair loss and muscle weakness—you empower yourself to be an advocate for your pet’s health.

If you suspect your dog may be exhibiting symptoms of hyperadrenocorticism, do not delay. Seek guidance from your veterinarian. Through a combination of accurate diagnosis, tailored treatment, and consistent home monitoring, you can help manage this chronic condition and restore a better quality of life for your cherished canine companion. Your dog’s well-being is worth the diligent effort.