Acromegaly (Hypersomatotropism) in Cats and Dogs

TL;DR. Acromegaly is a chronic hormonal disorder caused by a benign pituitary tumor that releases too much growth hormone, leading to physical changes, organ enlargement, and severe, insulin-resistant diabetes.

Acromegaly in cats typically causes a gradual enlargement of the head, abdomen, and body size.

What is it?

Acromegaly, scientifically known as hypersomatotropism, is a chronic endocrine disorder that primarily affects adult cats and, less commonly, dogs. The condition is caused by a functional adenoma—a benign, hormone-secreting tumor—located in the somatotropic cells of the pituitary gland. The pituitary gland is a tiny, pea-sized organ situated at the base of the brain, often referred to as the "master gland" because it regulates many of the body's hormone-producing systems.

When a tumor develops in these specific cells, it begins to secrete excessive, unregulated amounts of growth hormone (GH). In a healthy animal, growth hormone is released in controlled pulses to assist with cell regeneration, growth, and metabolic functions. However, when growth hormone is constantly flooded into the bloodstream, it triggers the liver to produce high levels of another hormone called insulin-like growth factor 1 (IGF-1). Together, these hormones cause two major physiological problems: abnormal tissue overgrowth and severe metabolic disruption.

First, the excess hormones promote the overgrowth of connective tissue, bone, and internal organs (viscera). This is known as the anabolic phase of the disease. Second, growth hormone acts as a powerful antagonist to insulin. It prevents insulin from doing its job, which is to usher glucose out of the bloodstream and into the cells for energy. This direct interference leads to profound insulin resistance and, eventually, a severe form of diabetes mellitus that is incredibly difficult to control with standard insulin therapy. Owners typically first realize something is wrong when their diabetic pet requires increasingly large doses of insulin without any improvement in blood sugar levels.

Causes & risk factors

In cats, the primary cause of acromegaly is almost exclusively a benign tumor (adenoma) of the pituitary gland. These tumors grow slowly but steadily, gradually increasing the amount of growth hormone in circulation.

There are several key risk factors and demographic patterns associated with this condition:

• Species and Gender: The disease is most frequently diagnosed in cats, particularly older male cats. According to clinical records, male mixed-breed cats that are eight years of age or older represent the classic demographic for this condition.
• Breed Predispositions: Mixed-breed cats (domestic shorthairs, medium hairs, and longhairs) have the highest reported incidence. American Shorthairs also show a documented predisposition.
• Canine Differences: In dogs, the condition is much rarer and behaves differently. While cats develop acromegaly due to pituitary tumors, dogs can develop growth hormone-related skin conditions. For example, Pomeranians are predisposed to a related syndrome historically referred to as growth hormone-responsive dermatosis, which is now classified under the umbrella of "Alopecia X." This condition causes symmetrical hair loss rather than the classic skeletal and organ enlargement seen in acromegalic cats.

Signs to watch for

Because acromegaly develops slowly and insidiously, the physical changes can be highly subtle in the early stages. Owners often do not notice the gradual alterations in their pet's appearance until the disease has progressed significantly.

"Because of the insidious onset and slowly progressive nature of the anabolic clinical signs, clients are often not aware of subtle changes in the appearance of their cat until the clinical signs are quite obvious. Anabolic changes in acromegalic cats include an increase in body size, enlargement of the abdomen and head, development of prognathia inferior, and weight gain..."
— A leading veterinary internal medicine reference

Common (Cardinal) Symptoms

• Polydipsia (Excessive Thirst): Your pet will drink significantly more water than usual due to the underlying diabetes.
• Polyuria (Excessive Urination): Increased water intake leads to frequent, heavy urination and a constantly soaked litter box or frequent accidents in the house.
• Polyphagia (Excessive Appetite): Despite high blood sugar, the cells are starving for energy because insulin is blocked, leading to a voracious, never-ending appetite.
• Weight Gain: Unlike typical diabetic pets who lose weight, acromegalic pets often gain weight or maintain their weight due to the growth-promoting effects of the excess hormones.
• Enlargement of the Head and Abdomen: The skull bones widen, and the belly becomes noticeably round and distended due to enlarged internal organs (such as the liver and kidneys).
• Prognathia Inferior (Underbite): The lower jaw grows faster and longer than the upper jaw, causing a pronounced underbite where the lower teeth protrude.
• Inspiratory Stridor and Stertor: Overgrowth of the soft tissues in the throat and upper airway leads to noisy breathing, snoring, or a harsh, high-pitched sound when inhaling.

Occasional Symptoms

• Lethargy: General sluggishness, lack of energy, or reluctance to play.
• Seizures: If the pituitary tumor grows large enough to press on the surrounding brain tissue (acting as a space-occupying mass), it can trigger neurological signs.
• Stupor: A state of near-unconsciousness or extreme mental dullness, also caused by the pressure of a large brain tumor.

Prognathia inferior, or a protruding lower jaw, is a classic physical sign of tissue and bone overgrowth in acromegalic cats.

How vets diagnose it

Diagnosing acromegaly requires a systematic approach. Most cases are suspected when a cat with confirmed diabetes mellitus fails to respond to standard, high-dose insulin therapy. If your vet is prescribing doses of insulin that would normally control diabetes in three or four average cats, and your pet's blood sugar remains dangerously high, acromegaly is a primary suspect.

To confirm the diagnosis, your vet will recommend specific diagnostic tests:

1. Serum IGF-1 Concentration: This is the most common screening test. Because growth hormone levels fluctuate wildly throughout the day, measuring GH directly can be unreliable. Instead, vets measure IGF-1, which remains stable in the bloodstream. A significantly elevated IGF-1 level in a poorly controlled diabetic cat strongly points to acromegaly.
2. Baseline Serum GH Concentration: This is the gold-standard test for directly measuring growth hormone levels. However, because of the pulsatile nature of hormone release and limited laboratory availability, it is often paired with or substituted by the IGF-1 test.
3. Advanced Brain Imaging (CT or MR Imaging): If the blood tests support a diagnosis of acromegaly, advanced imaging is the next step. A CT scan or MRI of the brain allows the vet to visualize the pituitary gland directly, confirming the presence, size, and location of the pituitary tumor.

"Identification of an increased serum IGF-1 concentration in a poorly controlled diabetic cat with insulin resistance and clinical features suggestive of acromegaly supports the diagnosis and provides justification for CT or MR imaging of the pituitary gland. A pituitary mass documented by CT or MR scanning... adds further evidence for the diagnosis..."
— A leading veterinary internal medicine reference

Advanced imaging, such as an MRI or CT scan, is used to confirm the presence of a pituitary tumor.

Treatment options

Managing acromegaly is complex and requires a multi-faceted treatment plan. The goals of therapy are to control the secondary diabetes, manage the physical symptoms, and, if possible, address the underlying brain tumor.

First-Line Medical Management

• Insulin Therapy: Your vet will prescribe pancreatic insulin to manage the diabetes. Because of the profound insulin resistance caused by the growth hormone, these pets often require exceptionally high doses of insulin. Your vet will need to monitor your pet's blood glucose curves closely to find a dose that keeps them safe from diabetic ketoacidosis without causing sudden, dangerous drops in blood sugar if hormone levels shift.

Radiation Therapy

• Radiation Therapy: This is the primary treatment aimed at the tumor itself. Stereotactic radiation therapy targets the pituitary mass to shrink the tumor and reduce hormone production. The clinical response to radiation is highly variable.

"However, the clinical response to radiation therapy is unpredictable and ranges from no response to a dramatic response, characterized by shrinkage of the tumor; elimination of hypersomatotropism; resolution of insulin resistance; and reversion to a subclinical diabetic state. Typically, tumor size and plasma GH and serum IGF-1 concentrations are decreased and insulin responsiveness improves after..."
— A leading veterinary internal medicine reference

Prognosis

The long-term prognosis for cats with acromegaly is highly unpredictable. For cats undergoing radiation therapy, some experience a dramatic improvement where the tumor shrinks, growth hormone levels drop, and the insulin resistance resolves completely, allowing the cat's diabetes to go into temporary remission. However, even with successful radiation, diabetes or insulin resistance frequently recurs six months or longer after the treatment is completed.

Without radiation therapy, the prognosis depends on how well the secondary diabetes can be managed with high doses of insulin, and whether the pituitary tumor grows large enough to cause neurological signs like seizures or stupor. Many cats can live for several years with supportive care, but they require intensive, lifelong monitoring and high-dose insulin management.

For dogs with the related condition, Alopecia X (growth hormone-responsive dermatosis), the prognosis is vastly different. Because this is primarily a cosmetic skin condition rather than a systemic tumor-driven disease, the long-term prognosis is excellent, and many dogs live normal lifespans even without active treatment.

"FIG 49-4... Endocrine alopecia in a 6-year-old Pomeranian with suspected adult-onset, growth hormone (GH)-responsive dermatosis; this is a presumptive diagnosis that is now included in the syndrome called Alopecia X... For these dogs the long-term prognosis is good, even without treatment."
— A leading veterinary internal medicine reference

Prevention

Because acromegaly is caused by a spontaneous, benign tumor of the pituitary gland, there are currently no known prevention strategies. It is not caused by diet, lifestyle, or environmental factors. The best course of action is early detection. If your pet is diagnosed with diabetes, ensure you work closely with your vet to monitor their response to insulin. Recognizing insulin resistance early allows for faster diagnostic testing and a wider range of management options before the physical changes and organ damage become advanced.

When to call your vet

You should contact your veterinarian if you notice any of the following signs in your pet:

• Your diabetic pet is drinking and urinating excessively despite receiving their prescribed insulin doses.
• Your pet is constantly hungry but you notice physical changes like a widening head, a swollen abdomen, or a protruding lower jaw.
• Your pet begins snoring loudly or making harsh, whistling noises when breathing.

Seek emergency veterinary care immediately if your pet experiences any of the following neurological red flags:

• Seizures (uncontrolled shaking, paddling of limbs, or loss of consciousness)
• Stupor (extreme mental dullness, inability to wake up, or lack of response to your voice)
• Sudden collapse or profound weakness

For specific breeds

If you own a Pomeranian, it is important to understand the distinction between feline acromegaly and canine growth hormone-responsive dermatosis (Alopecia X). While both involve growth hormone pathways, Pomeranians do not typically develop the life-threatening pituitary tumors or severe insulin resistance seen in cats. Instead, they may develop symmetrical hair loss on their trunk and thighs while sparing the head. This condition is primarily cosmetic and does not impact their overall lifespan or quality of life.

For owners of American Shorthairs or mixed-breed cats, being vigilant about the signs of diabetes (increased thirst, urination, and appetite) and noting any subtle changes in their facial structure or breathing sounds as they age (8 years or older) can lead to an earlier diagnosis and a better management plan.

Sources

• Internal Medicine, 5th Edition, pages 756, 757, 759.