Diagnosis: The tentative diagnosis is severe insulin-resistant Diabetes Mellitus, with a very high suspicion for Feline Acromegaly (Hypersomatotropism) as the underlying cause. The combination of an older male cat with difficult-to-control diabetes requiring a massive insulin dose (> 1-2 U/kg), coupled with physical changes like weight gain, a broad head, and enlarged paws, is classic for this condition.
DDx: Other causes of insulin resistance in cats must be considered. These include hyperadrenocorticism (Cushing's disease), chronic inflammation or infection (e.g., severe dental disease, urinary tract infection), pancreatitis, or problems with insulin handling and administration by the owner. However, the physical changes in this cat make acromegaly the top differential.
Diagnostic tests:
Screening Test: The single most useful screening test is measuring the serum concentration of Insulin-like Growth Factor 1 (IGF-1). GH from the pituitary tumor stimulates the liver to produce IGF-1, which will be markedly elevated.
Result: The cat's serum IGF-1 was 1,850 ng/mL [Reference: < 1,000 ng/mL]. This result is highly suggestive of acromegaly.
Definitive Diagnosis: Advanced imaging is required to identify the pituitary tumor. Contrast-enhanced CT or MRI of the head is the gold standard.
Result: A CT scan revealed a distinct, contrast-enhancing mass in the pituitary fossa.
Ancillary Diagnostics: An echocardiogram was recommended to investigate the heart murmur and screen for hypertrophic cardiomyopathy, a common secondary complication.
Definitive diagnosis: Acromegaly secondary to a functional pituitary adenoma, causing severe insulin resistance and Diabetes Mellitus.
Treatment: Treatment is focused on managing the diabetes and, if possible, addressing the pituitary tumor.
Diabetes Management: The immediate goal is to control the clinical signs of diabetes. This cat will require continued high-dose insulin therapy, potentially increasing the glargine dose even further under careful monitoring. The goal is to keep the blood glucose below the renal threshold (~300 mg/dL) for most of the day to resolve the PU/PD, rather than achieving tight glycemic control.
Tumor Treatment (Definitive Care):
The gold standard for treating the pituitary tumor is radiation therapy, specifically stereotactic radiosurgery (SRS). This can shrink the tumor, reduce GH secretion, and may even lead to remission of the diabetes.
Medical management with a somatostatin analog (e.g., pasireotide) can be attempted but is often cost-prohibitive and has variable efficacy.
Monitoring: This cat requires lifelong monitoring. This includes regular glucose curves or fructosamine checks, monitoring for cardiac and renal complications, and watching for the development of neurologic signs (e.g., circling, altered mentation) that could arise from the expansion of the pituitary mass. For more information, see the
Merck Veterinary Manual
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