Cushing's disease in dogs: symptoms, tests and treatment

Overview

Cortisol is an essential hormone involved in regulating metabolism, the stress response and the immune system. There are two adrenal glands, located near the kidneys. Cortisol is produced in the adrenal cortex, the gland's outer layer.

Excessive cortisol activity changes the function of several organ systems at the same time. Another name for the condition is hyperadrenocorticism, meaning overactivity of the adrenal cortex.

Strictly speaking, Cushing's syndrome is the collective name for the condition arising from all these causes. In precise medical usage, Cushing's disease means the pituitary-dependent form. In everyday language, the same name is also used for other forms.

An adrenal tumour can be either benign or malignant. Spread of adrenal tumours to other organs has been reported in about 14–50% of cases.

Causes

The pituitary hormone ACTH normally controls cortisol production by the adrenal glands. A pituitary tumour, usually benign, may release too much of this signalling hormone. Both adrenal glands enlarge in response to the continuous stimulation. The pituitary-dependent form accounts for about 80–85% of naturally occurring cases.

An adrenal tumour can produce cortisol independently of pituitary control. Estimates of the proportion of adrenal-dependent cases are 10–15% or 15–20%. The healthy adrenal gland on the opposite side may then shrink because ACTH secretion is reduced.

A third cause is long-term corticosteroid medication, resulting in treatment-induced Cushing's syndrome. Topical eye and ear drops can also be a source of hormone exposure. In this situation, the dog's own cortisol production may be reduced despite the high overall corticosteroid effect.

Risk factors

The condition is most often diagnosed in middle-aged and older dogs. Pituitary-dependent disease mainly occurs at around 7–12 years of age. However, any breed can be affected.

The pituitary-dependent form is seen particularly in small breeds, such as Poodles, Dachshunds and Yorkshire Terriers. The adrenal-dependent form is relatively more common in larger dogs.

Findings on the influence of sex are inconsistent. In some studies, about 75% of affected dogs were female, but other studies found no association.

The reported median body weight of dogs with adrenal-dependent disease is 18–20 kilograms. This is not a minimum weight required for the disease to occur.

By August 2023, one breed club's breeding committee had received reports of 39 Finnish Lapphunds with Cushing's disease. The number reported is not a disease prevalence figure for the whole breed.

Typical symptoms

A combination of signs raises suspicion, but tests are needed to distinguish other conditions with similar signs. Changes you may notice at home include:

• Increased drinking and larger amounts of urine.
• Urinating indoors or needing to go out at night.
• Constant hunger and more determined searching for food.
• More panting than usual and reduced tolerance of heat.
• A noticeably rounded, sagging abdomen.
• Loss of muscle and weakness.
• Thinning hair on both sides, especially the flanks and tail.
• Thin or darkened skin and more blackheads than usual.
• Slow-healing wounds or repeated skin infections.

Increased drinking and urination occur in about 80–90% of patients. Hard calcium deposits may also develop in the skin. Vomiting and diarrhoea are not usual direct signs of excess cortisol.

Complications and consequences

Excess cortisol can be associated with urinary tract and skin infections. A urine culture may reveal infection even when no white blood cells are visible in the urine. Blood pressure may also rise. About 10% of dogs with Cushing's also have diabetes.

Possible serious associated problems include blood clots and a gallbladder mucocele, a build-up of mucus in the gallbladder. An enlarging pituitary tumour can press on brain tissue and cause neurological signs.

With medication for the pituitary-dependent form, average survival is about 2–2.5 years. After pituitary surgery or radiotherapy, the average is about 2–5 years. These figures describe groups of patients, rather than the time an individual dog has left.

Average survival after surgery for an adrenal tumour is reported as 1.5–4 years. With medication, the corresponding average is about one year. Estimates of mortality associated with surgery are 10–25%, although figures of 14–60% have also been reported for mortality within one month. Assessment of surgical risk takes account of tumour size and the dog's other health conditions.

Diagnosis at the vet

The vet combines the symptom history, physical examination, basic laboratory tests and hormone test results. Previous corticosteroid products are reviewed, including eye and ear drops. Blood tests assess blood cells and internal organ function. A urine sample is checked for concentration and possible signs of infection.

A raised liver enzyme ALP is a common finding in dogs. In a dog without symptoms, this alone does not mean that hormone tests for Cushing's are needed. Another acute illness and stress can distort test results. Testing may be delayed for at least 2–4 weeks after an acute illness has improved.

A common first-choice test is the low-dose dexamethasone suppression test, or LDDST. Cortisol is measured initially and at four and eight hours after the test substance is given. The test examines how well the mechanism that suppresses cortisol production works.

The ACTH stimulation test is particularly useful for investigating the treatment-induced form and monitoring treatment safety. However, the result may be normal with adrenal tumours: false-negative rates of up to 41%, or almost 50%, have been reported.

A low urine cortisol-to-creatinine ratio makes Cushing's very unlikely. A high result requires further tests to confirm the diagnosis. The sample is preferably collected at home, because stress at the practice can increase the ratio.

Abdominal ultrasound assesses the structure of the adrenal glands and other abdominal organs. The blood ACTH concentration helps distinguish pituitary-dependent from adrenal-dependent disease. A CT or MRI scan of the brain may be needed to assess a pituitary tumour.

Partial suppression of cortisol during the LDDST can identify the disease as pituitary-dependent. However, a lack of suppression does not prove there is an adrenal tumour. Up to 35% of pituitary-dependent cases cannot be distinguished in this way.

A hormone test may need repeating if the result is unclear and symptoms persist. In a borderline case, the LDDST may be repeated after 2–3 months if suspicion remains strong. An abdominal CT scan is also usually needed when planning surgery for an adrenal tumour.

Urine culture helps identify bacterial infection and, when necessary, select a suitable antimicrobial medicine. Measuring urine protein adds to the assessment of kidney health. Chest X-rays may be taken if the dog has respiratory signs.

Treatment

The vet prescribes treatment and adjusts medication according to monitoring results. Treatment aims to reduce symptoms and complications associated with the disease. Concurrent conditions are treated separately.

Trilostane reduces cortisol production by the adrenal glands. It is used for both pituitary-dependent and adrenal-dependent disease. Medication can control symptoms, but the tumour remains in place. Medication is usually lifelong.

The preferred treatment for an adrenal tumour, where possible, is surgical removal. Temporary cortisol replacement may be needed afterwards. Surgical removal of the pituitary gland is another treatment option. Radiotherapy may be used for a large pituitary tumour causing neurological signs.

Excessive suppression of hormone production can cause dangerous adrenal insufficiency. Trilostane can rarely cause permanent damage to the adrenal glands. The dog's appetite, alertness and other signs are therefore assessed alongside cortisol measurements.

The first check is often carried out about ten days or two weeks after treatment starts. Once treatment is stable, later checks are usually every 3–6 months. There is no single monitoring programme suitable for every dog. Many general symptoms improve within a few weeks, but skin and coat recovery often takes months.

After pituitary removal, both thyroid hormone and cortisol replacement may be needed. Surgery can also disrupt secretion of vasopressin, the hormone that regulates water balance. Medication to suppress cortisol production may still be needed for several months after radiotherapy.

Follow-up checks also assess internal organ blood results and electrolyte balance as needed. The ACTH test result does not always directly reflect how well symptoms are controlled. The amount of medication is therefore not changed on the basis of a single measurement alone.

Medication improves symptoms in about 90% of patients. However, not all skin changes always resolve completely. About 80% of dogs achieve remission after pituitary removal. Recurrence has been reported in about 11% of dogs that underwent this surgery.

Prevention

Naturally occurring Cushing's cannot be prevented with home measures. The risk associated with corticosteroid medication is reduced by using the lowest effective treatment chosen by the vet. Long-term corticosteroids may nevertheless be necessary to treat another condition.

Do not stop corticosteroids abruptly on your own initiative. In treatment-induced Cushing's syndrome, medication may be reduced gradually according to the vet's plan if the original condition allows.

Some breed-club guidance for Finnish Lapphunds recommends excluding an affected dog, their offspring, parents and full siblings from breeding. This recommendation concerns breeding choices within that breed and does not mean that all close relatives will develop the disease.

Important for dog owners

Keep water available at all times despite increased urination. Give your dog enough opportunities to go outside to toilet. Drinking does not decrease immediately in every dog after treatment begins.

Record water consumption over 24 hours, for example every 1–2 weeks, using the same bowl and measurement method. Also note changes in appetite, urination, panting and enthusiasm for exercise. These observations help the vet assess the response to treatment alongside laboratory results.

Attend the agreed follow-up appointments even when your dog seems well. Blood testing checks that treatment has not lowered cortisol too far. Keep written instructions for what to do if your dog starts vomiting, develops diarrhoea or loses their appetite.

Frequently asked questions

Is Cushing's disease the same as an adrenal tumour?

Not always: most naturally occurring cases are pituitary-dependent. In these cases, a tumour associated with the brain stimulates the adrenal glands through a hormone signal. An adrenal tumour causes a different form, in which the tumour itself releases cortisol. Distinguishing the forms is particularly relevant when considering surgery.

Can an ordinary blood test diagnose Cushing's?

An ordinary blood test can show changes consistent with the condition, such as raised ALP. Diagnosis also requires assessment of symptoms and an appropriate hormone test. Stress at the practice or another concurrent illness can also cause a false-positive hormone test result.

Does medication completely cure Cushing's?

Medication reduces excess cortisol and can improve a dog's quality of life. However, it does not remove the underlying tumour. Removing an adrenal tumour can cure the disease in some cases. The choice of treatment depends on factors including surgical risks and the dog's other conditions.

Can I restrict water for a dog that drinks a lot?

No: a dog with Cushing's must have free access to drinking water. Offer more frequent, calm opportunities to go outside if urine volume has clearly increased. Measuring water consumption helps monitor treatment, but do so without removing your dog's access to water.

What should I do if my dog starts vomiting while taking trilostane?

Contact a vet and withhold the next trilostane until you have instructions. Vomiting may be an adverse effect of the medicine. Treatment can also lower cortisol to a dangerously low level, so do not monitor a clearly unwell dog at home while waiting for the next routine check.