Hyperadrenocorticism

Hyperadrenocorticism, commonly known as Cushing's disease or Cushing's syndrome, is an endocrine disorder characterized by chronic excessive production of cortisol by the adrenal glands. Cortisol is a glucocorticoid hormone essential for numerous physiological functions including glucose metabolism, immune regulation, stress response, and maintenance of blood pressure. When cortisol levels remain abnormally elevated over extended periods, the resulting hormonal imbalance produces a constellation of clinical signs that affect virtually every organ system in the body.

The condition is one of the most commonly diagnosed endocrine disorders in middle-aged to older dogs, with most cases presenting in animals between eight and twelve years of age. The disease develops insidiously, with clinical signs often progressing so gradually that owners attribute them to normal aging rather than a treatable medical condition. This slow onset frequently results in delayed diagnosis, as the individual signs of hyperadrenocorticism, such as increased drinking and urination, are nonspecific and overlap with many other conditions of aging dogs.

Hyperadrenocorticism is classified into three distinct forms based on the underlying cause of cortisol excess. Pituitary-dependent hyperadrenocorticism, which accounts for approximately eighty to eighty-five percent of naturally occurring cases, results from a functional tumor of the pituitary gland that secretes excessive amounts of adrenocorticotropic hormone. Adrenal-dependent hyperadrenocorticism, comprising approximately fifteen to twenty percent of cases, arises from a functional tumor of one adrenal gland that autonomously produces cortisol independent of pituitary control. Iatrogenic hyperadrenocorticism, the third form, results from the prolonged administration of exogenous corticosteroid medications.

Understanding the specific type of hyperadrenocorticism present in an individual dog is essential for determining the appropriate treatment approach, as the therapeutic options and prognosis differ significantly between pituitary-dependent, adrenal-dependent, and iatrogenic forms. Accurate differentiation between these types requires a systematic diagnostic approach that combines hormonal testing with advanced imaging of the pituitary and adrenal glands.

The pathophysiology of hyperadrenocorticism centers on disruption of the hypothalamic-pituitary-adrenal axis, the hormonal feedback system that normally maintains cortisol production within a tightly regulated range. In a healthy dog, the hypothalamus releases corticotropin-releasing hormone, which stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone. ACTH in turn stimulates the adrenal cortex to produce cortisol. Rising cortisol levels exert negative feedback on both the hypothalamus and pituitary, reducing further ACTH secretion and completing the regulatory loop.

In pituitary-dependent hyperadrenocorticism, a functional adenoma of the pars distalis or pars intermedia of the pituitary gland produces ACTH in excess of normal regulatory control. These adenomas are typically microadenomas measuring less than ten millimeters in diameter, though a subset of dogs develop macroadenomas that can exceed ten millimeters and produce neurological signs through compression of adjacent brain structures. The autonomous ACTH production by the pituitary tumor drives bilateral adrenal cortical hyperplasia and sustained cortisol overproduction that overwhelms the normal negative feedback mechanism.

Adrenal-dependent hyperadrenocorticism results from a functional adrenocortical tumor, either an adenoma or carcinoma, that produces cortisol autonomously without requiring ACTH stimulation. The chronically elevated cortisol from the tumorous gland suppresses pituitary ACTH secretion through negative feedback, causing atrophy of the contralateral adrenal gland. This pattern of one enlarged, tumorous adrenal gland and one atrophied contralateral gland is diagnostically useful for distinguishing adrenal-dependent from pituitary-dependent disease on imaging studies.

Iatrogenic hyperadrenocorticism develops when exogenous glucocorticoid medications, such as prednisone, prednisolone, or dexamethasone, are administered at doses or durations sufficient to produce chronic cortisol excess. Dogs receiving long-term corticosteroid therapy for conditions such as allergic dermatitis, immune-mediated diseases, or inflammatory bowel disease are at risk for developing iatrogenic Cushing's syndrome. The clinical signs are indistinguishable from naturally occurring hyperadrenocorticism, but the treatment approach is fundamentally different, involving gradual tapering of the exogenous medication rather than medical or surgical management of an endogenous cortisol source.

Breed predisposition to pituitary-dependent hyperadrenocorticism suggests a genetic component to disease susceptibility, though the specific genetic variants involved have not been fully characterized. Small-breed dogs including Poodles, Dachshunds, and Yorkshire Terriers are overrepresented among pituitary-dependent cases, while larger breeds may be proportionally more likely to develop adrenal tumors. The reasons for these breed-specific patterns remain an area of ongoing investigation.

The clinical presentation of hyperadrenocorticism in dogs involves a wide array of signs that reflect the pervasive effects of chronic cortisol excess on multiple organ systems. The classic clinical triad of polyuria, polydipsia, and polyphagia, meaning increased urination, increased water consumption, and increased appetite, is present in the majority of affected dogs and is often the first abnormality noticed by owners. Dogs with Cushing's disease may drink several times their normal daily water intake and produce correspondingly large volumes of dilute urine, sometimes leading to house-soiling in previously well-trained animals.

Dermatological changes are among the most visually prominent manifestations of hyperadrenocorticism. Chronic cortisol excess produces progressive bilateral symmetric alopecia that typically begins on the flanks and trunk while sparing the head and limbs. The skin becomes noticeably thin, fragile, and inelastic, and may develop a dry, scaly texture. Hyperpigmentation of the skin, particularly on the ventral abdomen, is common. Calcinosis cutis, the deposition of calcium within the skin, produces raised, firm plaques that are pathognomonic for hyperadrenocorticism when present, though this finding occurs in only a minority of cases.

Musculoskeletal changes include progressive muscle wasting and weakness, particularly of the proximal limb muscles and abdominal musculature. The combination of muscle wasting, hepatomegaly from steroid-induced hepatopathy, and redistribution of body fat produces the characteristic pendulous, pot-bellied appearance that is a hallmark of advanced Cushing's disease. Affected dogs may exhibit exercise intolerance, difficulty rising from a recumbent position, and reluctance to climb stairs or jump onto furniture.

Respiratory signs in dogs with hyperadrenocorticism include panting, which is disproportionate to the ambient temperature or activity level and results from cortisol's effects on the respiratory center and redistribution of abdominal fat that impairs diaphragmatic excursion. Chronic cortisol excess also suppresses the immune system, predisposing affected dogs to recurrent urinary tract infections, skin infections, and opportunistic infections that may not respond as expected to standard antimicrobial therapy.

Less commonly recognized manifestations of hyperadrenocorticism include reproductive abnormalities such as testicular atrophy in intact males and persistent anestrus in intact females, behavioral changes including restlessness and apparent cognitive decline, and increased susceptibility to thromboembolism. Pulmonary thromboembolism is a serious and potentially fatal complication of Cushing's disease that results from cortisol-induced hypercoagulability and can produce acute respiratory distress in an otherwise stable patient.

Diagnosing hyperadrenocorticism requires a systematic approach that integrates clinical findings, routine laboratory data, and specific endocrine function tests. No single test is both perfectly sensitive and specific for the disease, so diagnosis typically relies on a combination of results interpreted in the context of the clinical presentation. The diagnostic process can be conceptualized in two phases: confirming the presence of hyperadrenocorticism and then differentiating between pituitary-dependent and adrenal-dependent forms.

Routine laboratory screening in dogs with suspected hyperadrenocorticism commonly reveals a characteristic pattern of abnormalities. The complete blood count often shows a stress leukogram with mature neutrophilia, lymphopenia, monocytosis, and eosinopenia. Serum biochemistry typically demonstrates elevated alkaline phosphatase, often markedly so due to the induction of a steroid-specific hepatic isoenzyme, along with elevated alanine aminotransferase, hypercholesterolemia, and mild to moderate hyperglycemia. Urinalysis reveals dilute urine with a specific gravity below 1.020, and concurrent urinary tract infection is identified in a significant proportion of cases.

The low-dose dexamethasone suppression test is considered the screening test of choice for hyperadrenocorticism by many endocrinologists. This test measures cortisol levels before and at four and eight hours following intravenous administration of a low dose of dexamethasone. In normal dogs, the exogenous dexamethasone suppresses ACTH production through negative feedback, resulting in cortisol levels below a diagnostic threshold at both time points. Dogs with hyperadrenocorticism fail to suppress cortisol adequately at the eight-hour time point, and specific patterns of suppression at the four-hour mark can help differentiate between pituitary and adrenal disease.

The ACTH stimulation test measures the adrenal glands' cortisol response to exogenous ACTH administration. While less sensitive than the low-dose dexamethasone suppression test for diagnosing naturally occurring hyperadrenocorticism, the ACTH stimulation test is the test of choice for diagnosing iatrogenic Cushing's syndrome and is widely used for monitoring treatment response. A cortisol level drawn one to two hours after ACTH injection that exceeds the reference range supports a diagnosis of hyperadrenocorticism.

Abdominal ultrasonography provides critical information for differentiating between pituitary-dependent and adrenal-dependent hyperadrenocorticism. Bilateral adrenal gland enlargement suggests pituitary-dependent disease with bilateral adrenocortical hyperplasia, while asymmetric adrenal enlargement with one enlarged gland and contralateral atrophy suggests an adrenal tumor. Advanced imaging with computed tomography or magnetic resonance imaging of the brain can identify pituitary masses and assess their size, which has prognostic significance for dogs with pituitary-dependent disease.

Medical management is the most common treatment approach for pituitary-dependent hyperadrenocorticism and involves the use of medications that either inhibit adrenal cortisol synthesis or destroy adrenocortical tissue. The two primary medical options are trilostane and mitotane, each with distinct mechanisms of action, dosing protocols, monitoring requirements, and side effect profiles. The choice between these medications depends on clinical circumstances, veterinarian experience, drug availability, and owner factors.

Trilostane is a synthetic steroid analog that reversibly inhibits 3-beta-hydroxysteroid dehydrogenase, an enzyme essential for cortisol biosynthesis in the adrenal cortex. This competitive inhibition reduces cortisol production without destroying adrenocortical tissue, making the drug's effects dose-dependent and reversible. Trilostane is currently the most widely prescribed medical treatment for canine hyperadrenocorticism worldwide. Starting doses typically range from one to two milligrams per kilogram administered once or twice daily with food, with subsequent dose adjustments based on clinical response and ACTH stimulation test results.

Monitoring during trilostane therapy requires periodic ACTH stimulation testing to ensure adequate cortisol suppression without over-suppression. The stimulation test is typically performed four to six hours after the morning trilostane dose, with the goal of achieving a post-ACTH cortisol concentration within a target range that controls clinical signs while maintaining sufficient adrenal reserve for physiological stress responses. Electrolyte monitoring is also important, as trilostane can occasionally produce iatrogenic hypoadrenocorticism with potentially life-threatening electrolyte imbalances.

Mitotane, also known as o,p'-DDD, is an adrenolytic agent that selectively destroys the zona fasciculata and zona reticularis of the adrenal cortex, the tissue layers responsible for cortisol and sex steroid production. Treatment involves an induction phase during which daily doses are administered until clinical signs of cortisol reduction appear, followed by a maintenance phase with less frequent dosing. Mitotane produces more durable cortisol suppression than trilostane but carries greater risk of adrenocortical necrosis, which can progress to adrenal insufficiency if dosing is not carefully monitored.

For iatrogenic hyperadrenocorticism, the primary treatment is gradual, carefully supervised tapering of the exogenous corticosteroid medication. Abrupt discontinuation of chronic corticosteroid therapy can precipitate life-threatening adrenal crisis due to suppression of the hypothalamic-pituitary-adrenal axis. Tapering protocols vary depending on the dose, duration, and type of corticosteroid being withdrawn, but generally involve slow dose reductions over weeks to months while monitoring for signs of adrenal insufficiency.

Surgical management of hyperadrenocorticism offers the potential for definitive cure in selected cases, particularly for adrenal-dependent disease caused by unilateral adrenal tumors. Adrenalectomy, the surgical removal of the affected adrenal gland, eliminates the source of autonomous cortisol production and can restore normal adrenal function through recovery of the previously suppressed contralateral gland. However, adrenalectomy is a technically demanding procedure with significant perioperative risks that requires careful patient selection and experienced surgical expertise.

Preoperative preparation for adrenalectomy is critical for minimizing surgical complications. Dogs with hyperadrenocorticism have impaired wound healing, increased susceptibility to infection, and heightened risk of thromboembolism, all of which must be addressed before, during, and after surgery. Medical management with trilostane for several weeks before surgery can improve the patient's metabolic status and reduce perioperative risk. Assessment for vascular invasion is essential for adrenal carcinomas, as tumor extension into the caudal vena cava or phrenicoabdominal vein significantly increases surgical complexity and risk.

The surgical approach to adrenalectomy depends on tumor size, location, and evidence of vascular invasion. Standard open adrenalectomy through a ventral midline or paracostal approach provides excellent exposure and allows direct assessment of the tumor and surrounding structures. Laparoscopic adrenalectomy has gained popularity for smaller tumors without vascular invasion, offering advantages of reduced tissue trauma, faster recovery, and lower morbidity in selected patients. Conversion to open surgery may be necessary if unexpected complications are encountered during the laparoscopic approach.

Postoperative management following adrenalectomy for adrenal-dependent hyperadrenocorticism must account for the suppressed state of the contralateral adrenal gland. Exogenous glucocorticoid supplementation is typically required for weeks to months until the remaining adrenal gland recovers functional capacity. Monitoring involves periodic ACTH stimulation testing to assess adrenal recovery and guide the tapering of supplemental corticosteroids.

Radiation therapy is an option for dogs with pituitary-dependent hyperadrenocorticism caused by pituitary macroadenomas that are producing neurological signs due to brain compression. Stereotactic radiation and conventional fractionated radiation protocols have been used to reduce pituitary tumor size and alleviate neurological deficits. While radiation does not typically normalize cortisol production, it can significantly improve quality of life in dogs with large pituitary tumors by reducing mass effect on surrounding brain structures. Concurrent medical management of the cortisol excess is usually required alongside radiation therapy.

Chronic cortisol excess produces widespread metabolic and immunological derangements that predispose dogs with hyperadrenocorticism to a variety of secondary complications. Recognizing and managing these complications is an integral part of comprehensive Cushing's disease care and can significantly influence the patient's quality of life and long-term outcome.

Urinary tract infections are among the most common complications of hyperadrenocorticism, occurring in forty to fifty percent of affected dogs. Cortisol-induced immunosuppression impairs the local and systemic immune defenses that normally protect the urinary tract from bacterial colonization, while the dilute urine produced by these dogs provides a favorable medium for bacterial growth. Importantly, the immunosuppressive effects of cortisol may mask the typical inflammatory signs of urinary tract infection, meaning that affected dogs may have significant bacteriuria without exhibiting obvious signs of discomfort, urgency, or hematuria. Regular urinalysis with culture is recommended for all dogs with hyperadrenocorticism.

Hypertension is present in a significant proportion of dogs with Cushing's disease and results from cortisol's effects on vascular tone, sodium retention, and the renin-angiotensin-aldosterone system. Sustained hypertension can damage target organs including the kidneys, heart, eyes, and brain, potentially producing proteinuria, left ventricular hypertrophy, retinal detachment, and neurological signs. Blood pressure monitoring should be performed at diagnosis and periodically throughout treatment, with antihypertensive therapy initiated when sustained hypertension is documented.

Diabetes mellitus develops as a concurrent condition in approximately ten percent of dogs with hyperadrenocorticism. Cortisol antagonizes the action of insulin and promotes gluconeogenesis, producing insulin resistance that can overwhelm the pancreatic beta cells' ability to maintain normoglycemia. When diabetes mellitus and hyperadrenocorticism coexist, controlling the cortisol excess is essential for achieving adequate glycemic regulation, as insulin resistance will persist until cortisol levels are normalized.

Thromboembolism represents a life-threatening complication of hyperadrenocorticism that results from cortisol-induced hypercoagulability. Elevated cortisol promotes increased production of coagulation factors, decreased fibrinolytic activity, and platelet hyperreactivity, creating a prothrombotic state. Pulmonary thromboembolism is the most clinically significant thrombotic complication and can present as acute respiratory distress, collapse, or sudden death. Dogs with hyperadrenocorticism that develop acute respiratory signs should be evaluated for thromboembolism using thoracic imaging, blood gas analysis, and D-dimer measurement.

Successful long-term management of hyperadrenocorticism requires regular monitoring to assess treatment efficacy, detect complications, and adjust therapy as the disease evolves over time. Dogs with Cushing's disease require lifelong medical management in most cases, and the monitoring protocols are designed to maintain cortisol levels within a therapeutic range that controls clinical signs while preserving adequate adrenal reserve for physiological stress responses.

The frequency of monitoring depends on the stage of treatment. During the initial dose-finding period for trilostane or mitotane therapy, ACTH stimulation tests and clinical assessments are typically performed at ten to fourteen day intervals until stable cortisol control is achieved. Once a maintenance dose is established, monitoring intervals can be extended to every three to four months for stable patients, though any change in clinical signs should prompt earlier evaluation.

Clinical monitoring between veterinary visits relies heavily on owner observation of the dog's daily water consumption, urination frequency, appetite, activity level, and general demeanor. Providing owners with a structured daily monitoring log that tracks water intake, meal consumption, and activity level creates a valuable record that helps the veterinary team identify trends and adjust treatment before clinical deterioration becomes evident. Owners should be educated about the signs of both inadequate cortisol control and excessive cortisol suppression.

Excessive cortisol suppression, or iatrogenic hypoadrenocorticism, is the most serious adverse effect of medical treatment for hyperadrenocorticism. Signs of adrenal insufficiency include lethargy, weakness, vomiting, diarrhea, decreased appetite, and in severe cases, cardiovascular collapse. Owners must understand these warning signs and know to discontinue the medication and contact their veterinarian immediately if they develop. Emergency glucocorticoid supplementation may be required to stabilize dogs experiencing adrenal crisis from treatment-related cortisol depletion.

Long-term management also involves periodic reassessment of concurrent conditions that may develop or worsen as the dog ages. Regular urinalysis with culture, blood pressure monitoring, fasting glucose measurement, and hepatic enzyme evaluation should be incorporated into the monitoring protocol. For dogs with pituitary-dependent hyperadrenocorticism, periodic neurological assessment is warranted to detect early signs of pituitary tumor growth that might benefit from radiation therapy or require adjustment of the overall management plan.

The prognosis for dogs diagnosed with hyperadrenocorticism depends on the underlying cause, the presence and severity of concurrent complications, and the response to treatment. With appropriate medical management, many dogs with pituitary-dependent hyperadrenocorticism enjoy good quality of life for extended periods, with median survival times of approximately two to three years from the time of diagnosis reported in several clinical studies.

Dogs with pituitary-dependent hyperadrenocorticism caused by microadenomas generally have the most favorable prognosis among the naturally occurring forms. These dogs typically respond well to medical management with trilostane or mitotane, with clinical signs improving within the first few weeks to months of treatment. Resolution of polyuria, polydipsia, and polyphagia is usually observed earliest, while dermatological changes including hair regrowth and skin normalization may take several months to become apparent.

Pituitary macroadenomas carry a more guarded prognosis due to the potential for tumor growth to produce progressive neurological deficits. Clinical signs of pituitary macroadenoma expansion include circling, altered mentation, vision loss, incoordination, and seizures. Dogs that develop neurological signs from pituitary tumor growth may benefit from radiation therapy, which can reduce tumor size and improve neurological function. Without intervention, expanding macroadenomas can produce life-threatening brainstem compression.

The prognosis for adrenal-dependent hyperadrenocorticism varies considerably based on tumor type and stage. Adrenal adenomas that are successfully removed through adrenalectomy carry an excellent long-term prognosis, with many dogs achieving complete cure. Adrenal carcinomas have a more variable outlook depending on the presence of metastatic disease and the feasibility of complete surgical excision. Dogs with non-metastatic adrenal carcinomas that undergo successful adrenalectomy can have survival times of one to three years, while metastatic adrenal carcinoma carries a significantly worse prognosis.

Overall quality of life during treatment is generally good for most dogs with hyperadrenocorticism. The progressive improvement in clinical signs following initiation of appropriate therapy is often dramatic and gratifying for both owners and veterinary teams. Dogs that were lethargic, excessively thirsty, and losing hair frequently return to more normal activity levels, water consumption patterns, and coat quality, demonstrating that effective management can substantially restore comfort and function despite the chronic nature of the disease.

Managing a dog with hyperadrenocorticism is a long-term commitment that requires consistent owner engagement, regular veterinary care, and attention to the unique needs created by chronic cortisol excess. While the diagnosis can feel overwhelming initially, most owners find that with proper treatment and monitoring, their dog can maintain a comfortable, enjoyable quality of life for an extended period.

Medication administration requires reliability and consistency, as both trilostane and mitotane must be given on schedule and in the correct doses to maintain stable cortisol control. Trilostane should always be administered with food to enhance absorption and reduce gastrointestinal side effects. Owners who manage their dog's medication schedule proactively, using reminders and establishing consistent routines, are more likely to achieve stable disease control and fewer treatment complications.

Dietary considerations for dogs with hyperadrenocorticism focus on supporting overall health while addressing the metabolic consequences of cortisol excess. A high-quality, moderate-protein diet that supports lean muscle mass can help counteract the muscle wasting associated with the disease. Because hyperadrenocorticism predisposes dogs to pancreatitis and hepatic lipidosis, diets that are moderate in fat content may be advisable. Fresh water should be available at all times, as restricting water intake in a dog with cortisol-induced polyuria can lead to dehydration.

Exercise modification may be necessary, particularly during the early treatment period when muscle wasting and weakness are most pronounced. Gentle, regular exercise helps maintain muscle mass, support cardiovascular health, and promote psychological wellbeing. However, vigorous or prolonged exercise should be avoided until the dog's condition has stabilized on treatment, as cortisol excess impairs exercise tolerance and increases the risk of soft tissue injuries in dogs with weakened musculoskeletal tissues.

Skin care deserves special attention in dogs with hyperadrenocorticism. The thin, fragile skin characteristic of the disease is vulnerable to tearing, bruising, and secondary infection from minor trauma. Owners should handle their dog gently, avoid tight collars or harnesses that could abrade the skin, and inspect the skin regularly for wounds, infections, or calcinosis cutis. Prompt treatment of any skin wounds or infections is important, as the immunosuppressive effects of cortisol impair normal wound healing and infection clearance.