You’re probably familiar with the concept that hormones are the body’s messengers. Released in one part of the body, they travel to another prompting some kind of response in a target organ. Hormones have many functions, including triggering hunger or thirst, influencing the storage or release of energy, and causing or halting growth. A disorder affecting growth hormone (GH), called acromegaly, was previously believed to be rare in cats but is now thought to be more common than initially suspected.

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What is Acromegaly?

A part of the brain known as the pituitary gland is responsible for producing several hormones that give messages to various organs within the body. Growth hormone (also called somatotropin) produced here tells the liver to produce a protein called insulin growth factor-1 (IGF-1). This protein leads to growth and multiplication of cells all over the body, as well as influencing the breakdown of fat stores.

In humans, an excess of GH production is referred to as acromegaly due to the changes in bone and body growth with the disease. In our pet cats, vets often borrow the term acromegaly to describe the same excess GH production. However the medical name for the condition is hypersomatotropism. In the UK, this condition is estimated to affect 1 in 800-1000 cats, most commonly senior male neutered cats.

The most common cause of acromegaly is a benign tumour growing in the pituitary gland. More specifically, a tumour that functionally affects the cells producing GH. This tumour increases the size of the gland, leading to more GH being produced than necessary. Increased GH leads to increased IGF-1 production by the liver.

How could Acromegaly affect my cat?

As acromegaly develops, it has two main influences on your cat’s body: a change in organ size and an effect on metabolism. Because IGF-1 stimulates cell growth, some of the classic signs of acromegaly in humans is gigantism. In cats, this is less dramatic, but a change in bone structure can lead to a broadening of the face, a change in teeth positioning, and enlargement of the paws or joints. The skin may look puffy, or the neck may appear thickened which may coincide with snoring.

An increase in organ size may not always be outwardly obvious. However, if the organs within the belly enlarge enough, this may make the abdomen look bigger. A change in heart muscle can increase the size of the heart. This increases the likelihood of developing heart failure. Symptoms of this generally relate to an altered breathing pattern: an increased rate or depth of breathing in the early stages; open-mouth breathing or distressed and difficult breathing in the later stage. More rarely, the tumour itself can push on other parts of the brain impacting vision and movement.

GH has an additional effect on the body: it counteracts the effect of insulin

Increased GH production reduces the effectiveness of insulin. As a result, almost all cats that develop acromegaly, have diabetes mellitus (diabetes for short). For some cats, the only symptoms they will develop are those associated with diabetes. Because insulin can no longer control the level of blood glucose in the body, chronic high glucose causes hunger, thirst and an increase in urination.

In the UK, diabetes is estimated to affect 1 in 230 pet cats. Most often this occurrence is influenced by diet, weight and low activity. However, recent studies have shown that a quarter of diabetic cases are potentially linked to acromegaly. With medical insulin being the main therapy for feline diabetes, too much GH stops not just natural insulin from working, but also medical insulin. The effect is a diabetic cat that needs unusually high doses of insulin to control their diabetes, or an overall poor response to insulin therapy.

How is Acromegaly Diagnosed?

Your vet may suspect your cat has acromegaly if it has been diagnosed with diabetes but fails to respond to treatment. As there are more common reasons for reduced treatment response (including urinary tract infections and unmanaged dental disease), these need to be ruled out first. Obvious changes in body size or structure may add to the suspicion but not all affected cats show this.

There is no commercially available test to measure GH but your vet can measure IGF-1 via a blood test. This is an imperfect test though, as changes to IGF-1 levels occur for many reasons. Excessively high levels will usually suggest acromegaly, however normal levels, low levels or mildly high levels do not entirely rule out the disease.

If the suspicion for acromegaly remains following a blood test, then an MRI or CT scan of the brain can check for evidence of a pituitary tumour. The combination of high IGF-1 and a pituitary tumour on imaging gives the diagnosis. Where there are concerns for the health of other organs resulting from the acromegaly, an ultrasound may also be recommended, especially to assess the function of the heart.

How is Acromegaly Treated?

Acromegaly is managed in two ways: treat the symptoms or treat the disease. The most frustrating symptoms to both cat and owner are those linked to the diabetes. So symptomatic treatment involves treating the diabetes. This is usually in the form of once or twice a day insulin injections. Your vet can train you how to do this and it can quickly become a routine thing to do at home. Additionally, at-home monitoring of blood glucose levels in response to insulin injections is an important part of management.

Treatment of the diabetes alone can be difficult due to the reduced responsiveness to insulin therapy. The higher doses of insulin often required with acromegaly-induced diabetes make it a costly condition to treat. Even if the diabetes is controlled, the diabetic symptoms, especially a ravenous hunger, may remain. This incessant hunger can become problematic in some households.

The alternative is to treat the pituitary gland

Human medications have been used in cats to block GH production. However, they are potentially less effective in cats and can be expensive. Radiotherapy and surgery on the tumour are deemed to have a higher success rate. Both of these will involve your vet referring you and your cat to a specialist centre.

Radiotherapy has a high success rate at improving symptoms and stopping the disease. It is a specialised cancer treatment requiring multiple general anaesthetics over multiple weeks to administer. This may be stressful for your cat and may involve travel to a specialist centre away from your area. It is important to discuss the costs of this therapy before starting. Relapses can occur after radiotherapy, and the treatment itself may have unintended consequences on the eye or on other hormones produced in the area.

Surgery involves removing the pituitary gland in its entirety, taking the tumour with it. As a specialist procedure it involves intensive care for days after surgery. This treatment can improve heart health, as well as curing the acromegaly. However, it involves removing tissue responsible for producing other key hormones, resulting in the creation of new hormone deficiencies. These new conditions can be managed with medication and are considered a better quality of life for the cat than chronically uncontrolled diabetes.

Conclusion

Previously considered a rare disease, acromegaly (hypersomatotropism) is likely more common than initially believed. If your vet suspects your cat has this condition, they can recommend several tests to rule out other issues, as well as to confirm a diagnosis of acromegaly. Management is possible but can be costly and is not without risks. Discussing financial and welfare concerns with your vet at all stages of management can ensure the best treatment route is taken for your situation and your cat’s needs.

Further Reading:

  • Gouvêa, F.N., Pennacchi, C.S., Assaf, N.D., Branco, L.O., Costa, P.B., dos Reis, P.A., and Borin-Crivellenti, S. (2021) ‘Acromegaly in dogs and cats’ Annales d’Endocrinologie. 82(2), pp. 107–111.
  • Scudder, C. and Niessen, S. (2016) ‘Acromegaly in cats’The Veterinary Nurse. 7(4), pp. 224–230.

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