Excessive Growth Hormone (Somatotropin) in Cats

Acromegaly in Cats

Acromegaly is a rare syndrome resulting from the excessive production of the growth hormone somatotropin by tumors in the anterior pituitary gland of adult cats. Clinical signs of this syndrome are a result of the hormone's direct catabolic (breaking down) and indirect anabolic (building up) effects. 

The anabolic effects, meanwhile, are mediated by somatomedin C (insulin-like growth factor I), which is secreted by the liver in response to growth hormone stimulation. The excessive levels of somatomedin C, however, promote protein synthesis and growth in a variety of tissues such as the bone cartilage, soft tissues, especially in the head and neck region. Eventually these abnormalities in joint cartilage growth and metabolism alter normal joint biomechanics, which can lead to degenerative joint disease. 

Somatotropin also fights the action of insulin, which eventually causes pancreatic cell exhaustion and permanent diabetes mellitus.

Symptoms and Types

Initially, signs relate to unregulated diabetes mellitus. As the disease progresses, signs of heart failure, kidney failure, or central nervous system abnormalities caused by tumor expansion develop, including:

• Increased appetite (polyphagia)
• Excessive drinking (polydipsia)
• Excessive urinating (polyuria)
• Broadening of the facial features and lengthening of the lower jaw are common
• Weight loss (initially), followed by weight gain due to increased bone and soft tissue mass
• Systolic heart murmurs
• Seizures and/or other central nervous system signs

Causes

The hypersecretion of the growth hormone somatotropin by an anterior pituitary tumor.

Diagnosis

You will need to give a thorough history of your cat’s health to your veterinarian, including the onset and nature of the symptoms. He or she will then perform a complete physical examination as well as a complete blood count, biochemistry profile, urinalysis, and electrolyte panel.

Other diagnostic exams include X-rays, echocardiogram, and CT (computed tomography) and MRI (magnetic resonance imaging) scans. CT and MRI scans are used to locate the pituitary mass. X-rays, meanwhile, often reveal an enlarged heart and sometimes fluid in the lungs, especially if left-sided congestive heart failure has already developed. And an echocardiogram will confirm heart abnormalities.

A radioimmunoassay for plasma somatomedin C is available at Michigan State University -- which can confirm the elevated plasma levels associated with acromegaly -- but this is often impractical.

Treatment

Often, the goal is to treat and control the secondary diseases that develop following prolonged growth hormone hypersecretion (e.g., diabetes mellitus, heart failure, and kidney failure). However, there have been some successful attempts at treating the acromegaly.

In one study, for example, cobalt radiotherapy was used in which six out of seven acromegalic cats showed permanent or temporary resolution of insulin resistance following therapy. In another case, the surgical removal of the pituary tumor by freezing (cryohypophysectomy) also showed success. The cat slowly regained normal plasma somatomedin C levels and the diabetes mellitus resolved after two months.

Consult your veterinarian for the best course of treatment for your animal.

Living and Management

Your veterinarian will schedule follow-up appointments with you to treat your pet's secondary complications, as necessary.  Unfortunately, cats are usually euthanized or die because of complications associated with congestive heart failure, kidney failure, and/or progressive central nervous system signs (seizures, etc.).  Reported survival times following diagnosis range from 4 to 42 months, with a median of 20 months.