CONDITION
Pituitary Macroadenoma
A pituitary macroadenoma is a relatively large, typically benign growth in the pituitary gland — a small structure at the base of the brain that produces hormones influencing metabolism, growth, and reproduction. The term 'macro' describes a pituitary tumour that has become large enough to visibly enlarge the gland relative to the brain around it — rather than a fixed size in centimetres, this is judged using the pituitary's height relative to the brain (the P:B ratio, with values above roughly 0.31 indicating enlargement) or breed-adjusted height thresholds — and such tumours may exert physical pressure on surrounding brain tissue as well as disrupt normal hormone production. This can lead to effects both from excess hormone release and from compression of nearby structures. In dogs, owners often become aware of the possibility through changes in drinking and urination, appetite, energy, or behaviour, or because investigations for another concern have revealed an abnormality on blood tests or imaging. When the growth itself causes signs, the most common are vague ones: dullness, lethargy and a reduced appetite or refusal of food. Circling, seizures and loss of vision occur, but less often. In cats, the most common functional pituitary tumour instead produces excess growth hormone (acromegaly), which typically comes to light as diabetes that needs unusually high insulin doses. The presentation depends on which hormones are affected, how large the growth has become, and which adjacent structures are influenced. This page explores the signals that may be observed, the mechanisms underlying those signals, the investigations used to characterise the growth and its effects, and the range of approaches that exist — from monitoring and hormone management to radiation and surgery. Each section is written to help you understand what may be happening and what conversations may follow.
Why this matters now
Pituitary macroadenomas tend to be identified in middle-aged to older dogs, often between eight and twelve years of age, though younger animals can occasionally be affected. In UK primary-care data, Bichon Frises, Border Terriers and Miniature Schnauzers had the highest odds of Cushing's syndrome overall, and among dogs with cortisol excess one study found French Bulldogs more likely than other breeds to have a large pituitary tumour, though the condition can arise in any breed or cross. In cats, the growth hormone-secreting form is typically identified in middle-aged to older diabetic cats. There is no established nutritional or environmental trigger; the growth typically emerges spontaneously as part of age-related cellular change within the pituitary gland.
The tumour may grow slowly over months to years, and the clinical picture often develops in stages. Hormonal disturbances may precede signs of mass effect, or the reverse may occur, depending on which pituitary cell type is involved and the direction of tumour expansion. Some animals show a long plateau of subtle change, whilst others move more rapidly from early metabolic signs to neurological involvement. Progression varies widely between individuals, and the trajectory is not always predictable from initial presentation. Neurological decline is not always gradual. Bleeding into the tumour or loss of its blood supply (pituitary apoplexy) can cause abrupt deterioration, including marked dullness and collapse, and it can be fatal. In some dogs, neurological signs first appear or worsen after cortisol-lowering treatment is started. Excess cortisol also raises the risk of blood clots in the lungs (pulmonary thromboembolism), which can cause sudden laboured breathing and collapse over hours.
Signals & patterns
Early signals
Increased drinking and urination
An owner may notice the water bowl emptying faster than usual, or the dog asking to go outside more frequently, including overnight. This pattern can reflect hormonal changes affecting the kidney's ability to concentrate urine.
Increased appetite
The dog may seem persistently hungry, soliciting food more often or scavenging in ways that feel out of character. This is typically a consequence of excess cortisol. A large tumour can have the opposite effect: a reduced appetite or refusal of food is one of the more common signs of a macroadenoma pressing on the brain, so a dog whose hunger falls away is not necessarily improving.
Coat or skin changes
The coat may become thin, dull, or slow to regrow after clipping, and the skin may feel thinner or more fragile. These changes often accompany hormonal imbalances affecting hair follicle cycling and dermal structure.
Lethargy or reduced exercise tolerance
A dog may tire more easily on walks, seek rest more often, or show less enthusiasm for activities previously enjoyed. Hormonal shifts can alter muscle strength and energy regulation.
Panting at rest
Excess cortisol commonly causes panting, even when the dog is lying down and the room is cool. This panting is typically rhythmic, open-mouthed breathing without obvious strain. Laboured breathing is different and is not an expected hormonal effect: visible effort with each breath, flared nostrils, an extended neck, reluctance to lie down, or pale, grey or bluish gums. In a dog with excess cortisol, laboured breathing can reflect a blood clot in the lungs (pulmonary thromboembolism) or heart failure, both of which can worsen over hours, and the cause cannot be determined at home.
In cats: diabetes that is hard to control
A diabetic cat may keep drinking, urinating and eating heavily despite rising insulin doses, and may gain or hold weight rather than lose it. Over months, the face can broaden, the lower jaw can protrude, the paws can enlarge and breathing can become noisy. These changes reflect excess growth hormone from a pituitary tumour.
Later signals
Dullness, lethargy or reduced appetite
These vague signs are the most common neurological picture with a large pituitary tumour. The dog may seem flat and less responsive to familiar cues, sleep more, stare into space, appear disoriented in familiar settings, or eat less or stop eating. They are easily mistaken for ageing, and can reflect pressure on brain tissue as the tumour enlarges. In a dog receiving trilostane, the same signs, particularly alongside vomiting or diarrhoea, can instead mean the adrenal glands have been suppressed too far, which can progress to weakness and collapse. The two causes cannot be told apart at home.
Circling or head pressing
Repetitive pacing in one direction or pressing the head against walls or furniture may develop. These behaviours can indicate increased intracranial pressure or disruption of normal brain signalling.
Vision changes
An owner may observe the dog bumping into objects, hesitating in dim light, or showing reduced awareness of movement in the peripheral field. Vision loss is less common in dogs than dullness or reduced appetite, but compression of the optic nerves or visual pathways can produce these deficits.
Seizures
Episodes of collapse, paddling, loss of consciousness, or involuntary muscle activity may occur. These can arise when the mass disrupts electrical stability in adjacent brain regions, and they are less common than dullness or reduced appetite. A seizure lasting more than about five minutes, or repeated seizures without full recovery in between, can cause overheating, lasting brain injury and death. Several separate seizures within 24 hours (cluster seizures) can progress to this.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history of changes in drinking, urination, appetite, weight, behaviour, and any neurological signs, alongside a thorough physical examination. Blood and urine tests are often performed early to assess metabolic changes and hormone levels, which may point towards excess cortisol production in dogs, excess growth hormone in diabetic cats, or other endocrine disturbances. Imaging of the pituitary region and brain becomes important once the clinical picture suggests a mass, and specialised tests can help distinguish between different causes of hormone excess and determine the size and position of any growth.
Blood biochemistry and haematology
Endocrine function tests
Magnetic resonance imaging (MRI)
Physical and neurological examination
Computed tomography (CT)
Serum IGF-1 (in cats)
Options & trade-offs
Management is typically individualised, taking into account the size of the tumour, the presence and severity of hormonal disturbance, neurological signs, the owner's preferences, and the resources available. Many animals live with a combination of approaches rather than a single intervention, and the balance often shifts as the condition evolves. What is workable for one household and animal may be less suitable for another, and time frames for decision-making vary widely.
Medical management of hormone excess
In dogs, trilostane, the medicine authorised in the UK for this use, reduces cortisol production by blocking an enzyme in the adrenal glands, which can ease signs such as excessive drinking, urination, hunger and panting. It does not shrink the pituitary tumour itself but can improve quality of life when hormone excess is the dominant problem. Dosing is adjusted over weeks to months based on clinical response and repeat hormone testing. In cats with acromegaly, medical management usually centres on insulin for the diabetes, often at higher doses than usual, and does not address the growth hormone excess.
Trade-offs: Medical therapy requires ongoing monitoring, regular blood tests, and careful dose adjustment. It does not address mass effect, and neurological signs from tumour growth may still progress; in some dogs they first appear or worsen after treatment is started. In a dog on trilostane, vomiting, diarrhoea, lethargy, reduced appetite, shaking or weakness can be the first signs that the adrenal glands have been suppressed too far, and this can progress to electrolyte disturbance and collapse. Product guidance is that trilostane is stopped when these signs appear. Reactions of this kind can occur without warning and have rarely been fatal, and whether the signs reflect over-suppression cannot be judged at home without blood tests. Treatment is usually lifelong.
Radiation therapy
External beam radiation or stereotactic techniques aim to reduce tumour size and slow further growth by damaging the dividing cells within the adenoma. Treatment is typically delivered over several sessions under general anaesthesia at a specialist referral centre. The goal is to shrink the mass and relieve pressure on surrounding brain tissue, which may stabilise or improve neurological signs. In cats with acromegaly, it can also reduce insulin requirements.
Trade-offs: Radiation requires referral to a centre with the necessary equipment, involves multiple anaesthetics, and carries costs that may be substantial. Effects on tumour size develop over weeks to months, and not all tumours respond equally. Delayed side effects such as changes in brain tissue or pituitary function can occur, and some animals continue to need medical management of hormone excess alongside radiation.
Surgical removal (hypophysectomy)
Surgical removal of the pituitary gland via a transsphenoidal approach is performed at a small number of specialist centres. In cats with acromegaly, diabetic remission has been reported in a substantial proportion after surgery. The procedure aims to remove the tumour and halt both hormone excess and mass effect. Post-operative management includes lifelong hormone replacement for cortisol and thyroid deficiency, and sometimes diabetes insipidus requiring desmopressin.
Trade-offs: Surgery is technically demanding, carries anaesthetic and surgical risks, and is not widely available. Recovery can be prolonged, and complications such as bleeding, infection, or incomplete tumour removal may occur. Lifelong hormone supplementation is required, and not all animals are suitable candidates depending on age, concurrent disease, or tumour extent.
Monitoring and supportive care
In animals with mild or slowly progressive signs, or when other interventions are not practical, observation with periodic reassessment can be a reasonable path. Supportive measures may include managing symptoms such as increased thirst with access to water, addressing secondary infections, adjusting diet, or providing environmental modifications to accommodate vision or mobility changes.
Trade-offs: This approach does not alter the course of the tumour and signs may progress over time, sometimes unpredictably. It is most workable when quality of life remains acceptable. Periodic reassessment follows the slow course of the tumour; it does not cover abrupt changes such as collapse, prolonged or repeated seizures, or laboured breathing, which can develop over hours. It requires readiness to revisit the plan as the animal's condition evolves.
Common misconceptions
"If the tumour is benign, it will not cause serious problems."
Benign in this context means the tumour does not spread to other parts of the body, but its location within the confined space of the skull allows it to exert pressure on vital brain structures and disrupt hormone regulation. The effects can be substantial even without metastasis, and the term benign does not imply harmless.
"Once hormone levels are controlled with medication, the tumour is no longer a concern."
Medical management addresses the hormonal consequences but does not stop the tumour from growing. Mass effect can develop or worsen independently of cortisol levels, and neurological signs may emerge even when metabolic disturbances are well controlled. Imaging and clinical reassessment remain relevant over time.
"Radiation or surgery will cure the condition completely."
Both treatments aim to reduce tumour burden and improve or stabilise signs, but recurrence or incomplete response can occur, and some animals continue to need medical support for hormone imbalances. Outcomes vary, and the goal is often long-term control rather than permanent resolution.
Related conditions
Acromegaly in Cats
Acromegaly in cats arises from sustained excess growth hormone production, most often caused by a pituitary tumour—typically a macroadenoma—in the same gland affected in this condition. The two share a common origin and hormonal mechanism, though the clinical picture differs between species and depends on which hormones predominate.
Cushings Disease in Cats
Cushing's disease in cats is caused by excessive cortisol production, often driven by a pituitary tumour that releases adrenocorticotrophic hormone (ACTH). Pituitary macroadenomas can secrete ACTH, making Cushing's disease one possible hormonal consequence when this type of growth develops.
Brain Tumours
Pituitary macroadenomas are one specific type of growth within the skull, and the broader category of brain tumours includes many others that may present with overlapping neurological signs such as altered behaviour, circling, or changes in awareness. Understanding the distinctions helps frame the investigations and conversations that follow imaging.
Meningioma
Meningiomas arise from the membranes surrounding the brain rather than from the pituitary itself, but both can cause neurological signs through pressure on adjacent structures. Advanced imaging is often needed to distinguish between a pituitary macroadenoma and a meningioma in the same region of the skull.
Hydrocephalus
A large pituitary macroadenoma can, in some cases, compress or obstruct the pathways through which cerebrospinal fluid flows, potentially contributing to fluid accumulation within or around the brain. This overlap means that signs of raised intracranial pressure may prompt consideration of both the tumour itself and secondary fluid dynamics.
Cushings Disease in Dogs
Most dogs with Cushing's disease have a pituitary tumour that secretes ACTH, and in a proportion of them the tumour is large enough to be called a macroadenoma. The hormonal picture is shared, but a macroadenoma adds the possibility of neurological signs from pressure on the brain.
Pulmonary Thromboembolism
Excess cortisol from an ACTH-secreting pituitary tumour increases the tendency of the blood to clot, and a clot lodging in the lung vessels can cause sudden laboured breathing and collapse. This is one reason laboured breathing in a dog with this condition is not an expected hormonal effect.
Diabetes Mellitus in Cats
In cats, a growth hormone-secreting pituitary tumour opposes the action of insulin and underlies a considerable share of diabetes, particularly where high insulin doses fail to control it. Diabetes is often the sign that first brings feline acromegaly to attention.
Diabetes Insipidus
A large pituitary tumour can disrupt the release of antidiuretic hormone, causing central diabetes insipidus, and the same deficiency can follow surgical removal of the pituitary. Both lead to large volumes of dilute urine and marked thirst, which overlap with the signs of excess cortisol.
In a dog, the conversation often covers how cortisol excess is controlled, whether imaging of the brain is part of the picture, and how the effects of a large tumour would show themselves. In a diabetic cat, it often turns to whether high or rising insulin needs point to growth hormone excess. The tumour itself usually changes over months, but several of the problems linked to it develop over hours: laboured breathing from a clot in the lungs or heart failure, sudden dullness or collapse from bleeding into the tumour, a seizure lasting more than about five minutes or repeated seizures, and vomiting, diarrhoea, lethargy or weakness in a dog on trilostane, which can mean the adrenal glands have been suppressed too far. The causes of these changes cannot be separated at home.
Last reviewed: 13 September 2026 · Dr Alastair Greenway MRCVS