Feline acromegaly: The keys to diagnosis - Veterinary Medicine
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Feline acromegaly: The keys to diagnosis
Caused by excessive growth hormone secretion, this likely underdiagnosed endocrinopathy may be lurking in your feline patients—especially older, poorly controlled diabetic males. Here's a look at which diagnostic tests can help you detect it.


VETERINARY MEDICINE


DIAGNOSIS

No single test for the diagnosis of feline acromegaly exists. Diagnosing feline acromegaly starts with a clinical suspicion based on a thorough history, signalment, and clinical signs. Many of the abnormalities noted in the complete blood counts, serum chemistry profiles, and urinalyses of affected cats reflect concurrent diabetes mellitus, which stresses the need to carefully evaluate a patient's clinical history.

Common laboratory abnormalities associated with diabetes mellitus include hyperglycemia, increased liver enzyme activities (alanine transaminase, alkaline phosphatase), hypercholesterolemia, glucosuria, and isosthenuria. Also, since many cats with acromegaly are presented for evaluation of uncontrolled diabetes mellitus, azotemia and ketonuria are common.

Other common findings include erythrocytosis due to anabolic effects of growth hormone and IGF-1 and proteinuria secondary to glomerulonephropathy. Unexplained hyperphosphatemia and hyperglobulinemia have also been noted.1,2,4

Growth hormone assay

Serum growth hormone is often measured to help diagnose acromegaly in people; however, assays specific for feline growth hormone are not widely available. An assay using ovine growth hormone as the antigen has been validated for use in cats, but it is only available in Europe.5

Even if an assay were available, growth hormone concentrations alone may not be a reliable diagnostic test for acromegaly since growth hormone production is cyclic and concentrations may vary throughout the day.1 Relying on a single growth hormone measurement can be misleading.

Additionally, it has been shown that growth hormone concentrations may be elevated in diabetic cats that do not have acromegaly.6,7 This elevation may be due to the fact that the liver requires high levels of portal insulin to produce IGF-1, and in uncontrolled diabetics portal insulin concentrations may remain low, resulting in decreased IGF-1 production and, theoretically, decreased inhibition of growth hormone release.5,7

Finally, growth hormone concentrations may not be elevated early in the course of the disease but may increase significantly later.1

Serum IGF-1

Serum IGF-1 measurement is the most commonly used diagnostic test for feline acromegaly and is readily available in the United States. Unlike growth hormone, IGF-1 concentrations are less likely to fluctuate over the course of the day since most IGF-1 is protein-bound, giving it a longer half-life in the body. In addition, IGF-1 concentrations increase in response to chronically elevated growth hormone concentrations and are thought to be a reflection of growth hormone concentrations over the last 24 hours.1,8

A recent study evaluating IGF-1 concentrations in confirmed acromegalic diabetic cats, diabetic cats, and healthy cats found that acromegalic diabetic cats had significantly higher IGF-1 concentrations than diabetic and nondiabetic cats.9 This study concluded that serum IGF-1 concentration measurement is 84% sensitive and 92% specific for diagnosing feline acromegaly.

Just as with growth hormone, elevations in IGF-1 concentration alone may not definitively diagnose acromegaly in a cat. One study found that IGF-1 concentrations in nonacromegalic diabetic cats receiving long-term insulin treatment (> 14 months) had higher concentrations of IGF-1 than nondiabetic cats.8 It was proposed that insulin treatment allowed for beta cell regeneration and increased portal insulin, leading to elevations in IGF-1 concentrations.

In addition, another study revealed that untreated diabetic cats with acromegaly can have low to normal IGF-1 concentrations that increase after starting insulin therapy.7 The results of this study indicate that retesting IGF-1 concentrations a few weeks after starting insulin therapy or even after increasing insulin dosages in patients with suspected acromegaly that had low or normal IGF-1 concentrations may be warranted.


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Source: VETERINARY MEDICINE,
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