Cushing's Syndrome

Cushing's Syndrome, known medically as hyperadrenocorticism, is a hormonal disorder that develops when a dog's body produces excessive amounts of cortisol over a prolonged period. Cortisol is a steroid hormone produced by the adrenal glands, two small structures located near the kidneys. Under normal circumstances, cortisol plays essential roles in regulating metabolism, managing stress responses, controlling inflammation, and maintaining blood sugar levels. When cortisol is chronically overproduced, however, it disrupts virtually every organ system in the body, leading to a constellation of clinical signs that characterize this disease.

The condition is one of the most commonly diagnosed endocrine disorders in middle-aged and older dogs, with most cases occurring in animals over six years of age. Cushing's Syndrome develops gradually, and its early signs are often subtle and easily attributed to normal aging. Owners may notice their dog drinking more water, urinating more frequently, or developing a pot-bellied appearance long before a formal diagnosis is made. The insidious onset of symptoms means that many dogs live with undiagnosed Cushing's for months or even years before the condition is identified.

There are three recognized forms of Cushing's Syndrome in dogs. Pituitary-dependent hyperadrenocorticism (PDH) accounts for approximately 80 to 85 percent of naturally occurring cases and is caused by a tumor of the pituitary gland at the base of the brain that overproduces adrenocorticotropic hormone (ACTH), which in turn stimulates the adrenal glands to produce excessive cortisol. Adrenal-dependent hyperadrenocorticism (ADH) accounts for approximately 15 to 20 percent of cases and results from a tumor on one of the adrenal glands that autonomously produces cortisol independent of pituitary control. The third form, iatrogenic Cushing's Syndrome, occurs when a dog receives excessive doses of exogenous corticosteroid medications over an extended period.

Understanding which form of Cushing's Syndrome a dog has is critical for determining the most appropriate treatment strategy. While the clinical signs may appear identical regardless of the underlying cause, the therapeutic approach differs significantly between pituitary-dependent, adrenal-dependent, and iatrogenic forms of the disease.

Pituitary-dependent hyperadrenocorticism is by far the most common form and arises from a functional tumor in the pituitary gland, a pea-sized structure at the base of the brain that serves as the master regulator of many hormonal pathways. The majority of these pituitary tumors are microadenomas, small benign growths that are often too small to cause neurological symptoms but are capable of producing excessive amounts of ACTH. This hormone travels through the bloodstream to the adrenal glands, stimulating them to produce far more cortisol than the body requires. In a smaller percentage of cases, the pituitary tumor may be a macroadenoma, a larger growth that can expand to compress surrounding brain tissue and cause neurological signs such as behavioral changes, circling, blindness, or seizures.

Adrenal-dependent hyperadrenocorticism results from a tumor located directly on one of the adrenal glands. These tumors may be adrenocortical adenomas (benign) or adrenocortical carcinomas (malignant). Both types produce cortisol autonomously, without the normal regulatory feedback from the pituitary gland. Adrenal carcinomas tend to be larger, may invade surrounding structures including the vena cava and kidneys, and carry a risk of metastasis to the lungs, liver, and other organs. The distinction between benign and malignant adrenal tumors has significant implications for treatment planning and prognosis.

Iatrogenic Cushing's Syndrome develops as a consequence of long-term corticosteroid therapy prescribed for conditions such as allergies, immune-mediated diseases, or inflammatory disorders. Prednisone, prednisolone, dexamethasone, and other glucocorticoid medications, when administered at high doses or for extended periods, can mimic the effects of naturally elevated cortisol levels. The clinical presentation is identical to naturally occurring Cushing's Syndrome, but the treatment is fundamentally different, involving a carefully managed tapering of the exogenous steroid medication rather than the drug therapy used for PDH or ADH.

In rare cases, ectopic ACTH secretion from non-pituitary tumors has been reported in dogs, although this is far less common than in human medicine. The vast majority of canine Cushing's cases fall neatly into the pituitary-dependent or adrenal-dependent categories. Accurate identification of the cause through appropriate diagnostic testing is essential, as the treatment approach, expected response, and long-term prognosis differ substantially between forms.

The clinical signs of Cushing's Syndrome are numerous and reflect the widespread effects of excess cortisol on multiple body systems. The most frequently observed symptoms are polyuria (excessive urination) and polydipsia (excessive water consumption), which occur in the vast majority of affected dogs. Owners often report that their dog is emptying the water bowl repeatedly, seeking out unusual water sources, or having urinary accidents in the house despite previously being reliably house-trained. These changes occur because cortisol interferes with the kidney's ability to concentrate urine, leading to the production of large volumes of dilute urine.

Increased appetite, or polyphagia, is another hallmark sign and is among the most consistently reported symptoms. Dogs with Cushing's Syndrome may become ravenously hungry, beg incessantly for food, scavenge, or guard their food aggressively. This increased appetite, combined with cortisol's effects on fat distribution and muscle metabolism, contributes to the characteristic pot-bellied or pendulous abdominal appearance that is often one of the most visually striking features of the disease. The distended abdomen results from a combination of fat redistribution to the abdominal cavity, hepatomegaly (liver enlargement), and weakening of the abdominal musculature.

Dermatological changes are prominent and varied in dogs with Cushing's Syndrome. Bilateral symmetric hair loss that spares the head and extremities is a classic finding. The remaining coat may become dull, dry, and slow to regrow after clipping. The skin itself often becomes thin, fragile, and prone to bruising, tearing, and delayed wound healing. Calcinosis cutis, the deposition of calcium salts within the skin, may develop as firm, white or gritty plaques that can ulcerate and become secondarily infected. Recurrent skin infections and ear infections are common, as cortisol suppresses immune function.

Other common signs include panting (even at rest or in cool environments), lethargy and reduced exercise tolerance, muscle wasting and weakness (particularly noticeable in the hind limbs), and an increased susceptibility to urinary tract infections. Some dogs develop hypertension, which may contribute to kidney damage, retinal detachment, or cardiac complications over time. In intact female dogs, cessation of the estrous cycle may occur, and in intact males, testicular atrophy has been reported. The constellation of symptoms is often progressive, worsening over months as cortisol levels continue to rise unchecked.

Diagnosing Cushing's Syndrome requires a combination of clinical suspicion, routine laboratory findings, and specific endocrine testing. No single test is perfectly sensitive or specific for the disease, so veterinarians typically use a stepwise approach that begins with baseline blood work and urinalysis and progresses to specialized hormonal assays. The diagnostic process can be complex, and in some cases, repeated testing over time is necessary to confirm the diagnosis.

Routine blood work in dogs with Cushing's Syndrome frequently reveals a characteristic pattern of abnormalities. The most consistent finding is a markedly elevated alkaline phosphatase (ALP), which may be 5 to 40 times the normal range due to the induction of a cortisol-specific isoenzyme in the liver. Elevated cholesterol and triglycerides, mildly elevated blood glucose, and a stress leukogram (characterized by neutrophilia, lymphopenia, eosinopenia, and monocytosis) are also commonly observed. Urinalysis typically shows dilute urine with a specific gravity below 1.020, and urine culture may reveal a subclinical urinary tract infection.

The low-dose dexamethasone suppression test (LDDS) is considered the screening test of choice by many endocrinologists. A baseline cortisol level is drawn, a low dose of dexamethasone is administered intravenously, and cortisol levels are measured again at four and eight hours post-injection. In a normal dog, the exogenous dexamethasone suppresses the pituitary-adrenal axis, and cortisol levels drop significantly. In dogs with Cushing's Syndrome, cortisol levels fail to suppress adequately at the eight-hour mark. The LDDS test has a sensitivity of approximately 90 to 95 percent for detecting Cushing's Syndrome and can sometimes help differentiate between pituitary and adrenal forms based on the pattern of suppression at the four-hour time point.

The ACTH stimulation test is another widely used diagnostic tool and is the only test that can diagnose iatrogenic Cushing's Syndrome. A baseline cortisol sample is collected, synthetic ACTH (cosyntropin) is administered, and a post-stimulation cortisol level is drawn one to two hours later. An exaggerated cortisol response supports a diagnosis of naturally occurring Cushing's Syndrome, while a blunted response in a dog receiving corticosteroid medications confirms iatrogenic disease. The ACTH stimulation test has somewhat lower sensitivity than the LDDS test (approximately 80 to 85 percent) but is also used for monitoring treatment response.

Once Cushing's Syndrome is confirmed, differentiating between pituitary and adrenal forms is essential. Abdominal ultrasound is a valuable tool for this purpose, as it allows direct visualization of the adrenal glands. In pituitary-dependent disease, both adrenal glands are typically symmetrically enlarged. In adrenal-dependent disease, one adrenal gland is typically enlarged by a mass while the contralateral gland is atrophied due to suppression by the excess cortisol. Advanced imaging with CT or MRI may be pursued to evaluate the size of a pituitary tumor, particularly if neurological signs are present or if radiation therapy is being considered.

Treatment for Cushing's Syndrome varies depending on the underlying cause and must be carefully tailored to the individual patient. For pituitary-dependent Cushing's, which represents the majority of cases, medical management with oral medications is the standard approach. The two primary drugs used are trilostane and mitotane, each with distinct mechanisms of action, dosing protocols, and monitoring requirements.

Trilostane (brand name Vetoryl) has become the most widely prescribed medication for PDH in many countries. It works by inhibiting the enzyme 3-beta-hydroxysteroid dehydrogenase, which is required for cortisol synthesis in the adrenal glands. Trilostane is administered orally once or twice daily with food and produces a reversible reduction in cortisol production. Starting doses are typically based on body weight, and the dose is adjusted based on the results of ACTH stimulation tests performed at regular intervals. Most dogs show clinical improvement within the first few weeks of therapy, with reductions in water intake, urination, and appetite often being the earliest signs of response.

Mitotane (brand name Lysodren) is an older medication that works by selectively destroying the cortisol-producing layers of the adrenal cortex. Treatment with mitotane involves an initial loading phase during which the drug is administered daily until cortisol levels are reduced to the target range, followed by a maintenance phase with less frequent dosing. Mitotane is highly effective but carries a greater risk of serious side effects, including adrenal necrosis and the development of hypoadrenocorticism (Addison's disease) if too much adrenal tissue is destroyed. Close monitoring during the loading phase is essential, and owners must be thoroughly educated about the signs of cortisol deficiency.

For adrenal-dependent Cushing's caused by an adrenal tumor, surgical removal of the affected adrenal gland (adrenalectomy) is the treatment of choice when the tumor is resectable. Adrenalectomy is a major surgical procedure that carries significant risks, including hemorrhage from the vena cava and post-operative adrenal crisis, but it offers the possibility of a cure if the tumor is benign and completely excised. In cases where surgery is not feasible due to tumor invasion, metastasis, or the patient's overall health status, medical management with trilostane or mitotane can be used to control clinical signs.

Iatrogenic Cushing's Syndrome is managed by gradually tapering the exogenous corticosteroid medication under close veterinary supervision. Abrupt discontinuation of chronic steroid therapy can precipitate a life-threatening adrenal crisis, as the adrenal glands may have atrophied during the period of exogenous steroid administration and require time to resume normal function. The tapering schedule must be individualized based on the dose and duration of steroid therapy, and the underlying condition for which steroids were prescribed must be managed with alternative treatments.

Successful management of Cushing's Syndrome requires ongoing monitoring throughout the dog's life, as the disease is typically managed rather than cured with medical therapy. Regular veterinary visits, periodic blood testing, and close owner observation are all essential components of long-term management. The goal of treatment is to control clinical signs and cortisol levels while avoiding the complications of excessive cortisol suppression.

For dogs receiving trilostane, ACTH stimulation tests are typically performed at 10 to 14 days after starting therapy, at 30 days, at 90 days, and then every three to six months thereafter. The test should be performed four to six hours after the morning dose of trilostane to capture the time of peak drug effect. The target post-ACTH cortisol level is generally between 1.45 and 5.4 micrograms per deciliter, although clinical response should always be weighed alongside laboratory values. Dose adjustments are made based on the combination of test results and the dog's clinical signs.

For dogs on mitotane, monitoring during the loading phase is particularly critical. Owners are instructed to observe their dog closely for signs of cortisol deficiency, including loss of appetite, vomiting, diarrhea, lethargy, and weakness. If any of these signs develop, mitotane is discontinued immediately, and the veterinarian is contacted. An ACTH stimulation test is performed at the end of the loading phase and periodically during maintenance therapy. Blood chemistry panels and urinalysis should be checked regularly to assess liver and kidney function and to screen for secondary complications.

Beyond endocrine monitoring, dogs with Cushing's Syndrome benefit from regular screening for the complications associated with chronic cortisol excess. Urine cultures should be performed periodically, as these dogs are prone to silent urinary tract infections that may not produce obvious clinical signs. Blood pressure measurement is recommended, as hypertension is a common sequela that can damage the kidneys, eyes, and cardiovascular system. Monitoring for the development of diabetes mellitus is also important, as chronic cortisol elevation impairs insulin sensitivity and can precipitate overt diabetes in predisposed individuals.

Owners play a vital role in long-term management by tracking their dog's water intake, appetite, activity level, and urinary habits. Maintaining a daily log of water consumption and noting any changes in behavior or appearance can provide valuable information to the veterinarian at follow-up visits. Any sudden changes, particularly a dramatic decrease in appetite or energy level, should prompt immediate veterinary consultation, as these may indicate excessive cortisol suppression requiring urgent dose adjustment.

The prognosis for dogs with Cushing's Syndrome depends on the underlying cause, the presence of concurrent diseases, and the response to treatment. For dogs with pituitary-dependent disease managed with trilostane or mitotane, the prognosis is generally favorable, with many dogs achieving good control of clinical signs and enjoying a quality of life comparable to their pre-disease state. Published studies report median survival times of approximately two to three years following diagnosis, although some dogs live considerably longer with well-managed disease.

Quality of life typically improves substantially within the first few weeks to months of initiating treatment. Owners often report that their dog's excessive thirst and urination resolve first, followed by gradual improvements in energy level, coat quality, and muscle tone. The pot-bellied appearance may take several months to resolve fully, as fat redistribution and abdominal muscle strengthening occur slowly. Skin and coat changes, including hair regrowth, may take three to six months or longer to become fully apparent.

The prognosis for adrenal-dependent Cushing's caused by a benign adrenal adenoma is good to excellent if surgical removal of the tumor is successful. Dogs that undergo complete adrenalectomy for adenomas may be effectively cured, although post-operative supplementation with cortisol and mineralocorticoids may be necessary for a period while the atrophied contralateral adrenal gland recovers function. The prognosis for adrenal carcinomas is more guarded and depends on whether the tumor has invaded surrounding structures or metastasized. Malignant adrenal tumors that are completely resected may carry a reasonable prognosis, while those with metastatic disease have a poor long-term outlook.

Several factors can negatively influence prognosis. Dogs with large pituitary macroadenomas may develop progressive neurological signs that are difficult to manage. The development of concurrent conditions such as diabetes mellitus, pancreatitis, pulmonary thromboembolism, or serious infections can complicate management and reduce survival. Advanced age and the presence of other comorbidities also influence the overall outlook. Despite these potential complications, many dogs with Cushing's Syndrome respond well to therapy and maintain a good quality of life for an extended period with appropriate management.

Cushing's Syndrome shows clear breed predispositions, with certain breeds being significantly overrepresented in case studies and clinical populations. Poodles of all sizes (Toy, Miniature, and Standard) are among the most commonly affected breeds, along with Dachshunds, Boston Terriers, Boxers, and Beagles. Yorkshire Terriers, Staffordshire Bull Terriers, Jack Russell Terriers, and Labrador Retrievers also appear at higher frequency in many reports. While any breed can develop Cushing's Syndrome, these predispositions suggest an underlying genetic component to susceptibility.

Age is one of the most significant risk factors for naturally occurring Cushing's Syndrome. The disease is predominantly a condition of middle-aged to older dogs, with the vast majority of cases diagnosed in animals over six years of age. The median age at diagnosis is typically around 10 to 11 years. Cushing's Syndrome in dogs under six years of age is uncommon and should prompt careful investigation to confirm the diagnosis and rule out other conditions that may mimic its clinical presentation.

Body size appears to influence which form of Cushing's Syndrome a dog is likely to develop. Pituitary-dependent disease is more common in small to medium-sized breeds, while adrenal tumors occur with relatively greater frequency in larger breed dogs, although PDH remains the most common form across all sizes. There is no consistent sex predisposition reported in the veterinary literature, with both males and females affected at roughly equal rates in most studies, although some reports suggest a slight female predominance.

The iatrogenic form of Cushing's Syndrome can occur in any dog regardless of breed, age, or size, as it is entirely dependent on the administration of exogenous corticosteroids. Dogs with chronic allergic skin disease, immune-mediated hemolytic anemia, inflammatory bowel disease, or other conditions requiring long-term steroid therapy are at particular risk. The risk of iatrogenic Cushing's increases with higher doses, longer treatment durations, and the use of more potent glucocorticoid formulations. Veterinarians aim to use the lowest effective dose for the shortest possible duration to minimize this risk, and alternative anti-inflammatory or immunosuppressive medications are preferred when available.

Chronic cortisol excess in dogs with Cushing's Syndrome can lead to a range of serious complications that significantly impact health and quality of life. One of the most clinically important complications is an increased susceptibility to infections. Cortisol is a potent immunosuppressant, and dogs with uncontrolled Cushing's Syndrome are prone to bacterial skin infections (pyoderma), urinary tract infections, fungal infections, and respiratory infections. Urinary tract infections are particularly common and may be subclinical, meaning the dog shows no obvious signs of discomfort despite having a significant bacterial burden in the bladder.

Diabetes mellitus is a well-recognized concurrent condition in dogs with Cushing's Syndrome. Chronic cortisol elevation promotes insulin resistance and increases hepatic glucose production, and approximately 10 percent of dogs with Cushing's Syndrome develop overt diabetes. When both conditions coexist, management becomes significantly more challenging, as controlling cortisol levels is often necessary before insulin therapy can effectively regulate blood glucose. In some cases, diabetes resolves or becomes much easier to manage once Cushing's Syndrome is adequately treated.

Pulmonary thromboembolism (PTE) is one of the most feared complications of Cushing's Syndrome and represents a significant cause of morbidity and mortality. Dogs with hyperadrenocorticism are in a hypercoagulable state due to the effects of cortisol on coagulation factors, fibrinolysis, and vascular endothelial function. Clinical signs of PTE include sudden onset of severe respiratory distress, tachypnea, cyanosis, and collapse. PTE can be rapidly fatal and is often difficult to diagnose definitively ante-mortem, although CT angiography has improved detection rates in recent years.

Hypertension is present in a substantial proportion of dogs with Cushing's Syndrome, with some studies reporting prevalence rates of 50 to 80 percent. Sustained high blood pressure can cause damage to the kidneys (proteinuria and progressive renal disease), the eyes (retinal hemorrhage and detachment), the heart (left ventricular hypertrophy), and the brain (encephalopathy). Blood pressure monitoring should be a routine component of the management plan for any dog with Cushing's Syndrome.

Other notable complications include pancreatitis, which may occur as an acute or chronic condition in dogs with Cushing's Syndrome; gallbladder mucocele formation, which has been associated with hypercortisolism and can lead to gallbladder rupture and biliary peritonitis; ligament and tendon degeneration, including an increased risk of cranial cruciate ligament rupture; and calcinosis cutis, a disfiguring and uncomfortable condition in which calcium deposits form within the skin, leading to firm plaques that may ulcerate and become infected.

Owners should seek veterinary evaluation if they notice a progressive increase in their dog's water consumption and urination, particularly in a middle-aged or older dog. While these signs can have many causes, they are among the earliest and most consistent indicators of Cushing's Syndrome and warrant investigation. Similarly, a significant increase in appetite, unexplained weight gain or changes in body shape (particularly the development of a pot-bellied appearance), or progressive hair loss and skin changes should prompt a veterinary visit.

Dogs already diagnosed with and being treated for Cushing's Syndrome require immediate veterinary attention if they develop signs of cortisol deficiency, which can occur as a side effect of trilostane or mitotane therapy. Warning signs include sudden loss of appetite, vomiting, diarrhea, profound lethargy or weakness, shaking, and collapse. These signs may indicate that cortisol levels have been suppressed below the physiological minimum, creating a potentially life-threatening situation analogous to an Addisonian crisis that requires emergency supplementation with glucocorticoids.

Respiratory emergencies in dogs with Cushing's Syndrome should be treated with the utmost urgency. Sudden onset of labored breathing, rapid shallow respirations, blue-tinged gums, or collapse in a dog with known hyperadrenocorticism may indicate pulmonary thromboembolism, which is a medical emergency requiring immediate stabilization and treatment. Any acute change in respiratory effort should prompt an emergency veterinary visit without delay.

Regular follow-up appointments should be maintained even when a dog appears to be doing well on treatment. Cushing's Syndrome is a dynamic condition, and medication doses may need adjustment over time as the disease progresses or as the dog ages. Sudden recurrence of clinical signs such as increased drinking and urination after a period of good control may indicate that the current medication dose is no longer adequate, or that a concurrent condition has developed. Owners should feel empowered to contact their veterinarian whenever they observe changes in their dog's health or behavior, as early intervention in the management of this complex disease consistently leads to better outcomes.

For dogs not yet diagnosed but showing suggestive signs, owners should keep a log of daily water consumption, note any changes in urination patterns, document any skin or coat changes with photographs, and record appetite and activity levels. This information can be extremely valuable to the veterinarian during the diagnostic workup and helps establish the timeline and progression of symptoms.