Hypersomatotropism (HST) is the number one cause of insulin resistance in cats, with hypercortisolism (HC) occupying a more distant second place — distinguishing between them, and ruling out management-related causes first, drives the diagnostic approach.Veterinary Clin…

Before attributing insulin resistance to an endocrinopathy, management-related causes must be excluded. These include short insulin duration of action, inappropriate injection technique, and inappropriate insulin storage.Veterinary Clin… Only after these are addressed should endocrine screening proceed.

The clinical presentation of HST overlaps substantially with other causes of insulin resistance, making biochemical screening essential. Only approximately 25% of HST cats display the classic acromegalic phenotype of prognathia inferior and broadening of the face and paws.Journal of Feli… Features that raise suspicion without being pathognomonic include marked polyphagia, stertor or snoring, and persistently uncontrolled diabetes mellitus despite escalating insulin doses.Journal of Feli…+1 Insulin doses as high as 35 U q12h have been reported in acromegalic cats.AAHA Clinical G… Cats may lose weight initially but gain weight or maintain weight later in the disease course despite severe polyuria, polydipsia, and polyphagia.AAHA Clinical G… In contrast, HC in cats is more often associated with fragile skin and alopecia with dermatologic disease, features not typical of HST.AAHA Clinical G…

HST affects 15–32% of diabetic cats depending on the population studied, with reported prevalences of 24.8% in the UK, 17.8% in the Netherlands and Switzerland, 14.9% in Argentina, and 18–32% across multiple cohorts.Journal of Vete…+3 HC is considerably rarer in cats.Veterinary Clin… This prevalence difference alone justifies screening for HST before HC in any insulin-resistant diabetic cat.

Serum insulin-like growth factor-1 (IGF-1) is the diagnostic test of choice for HST screening and is recommended at the time of diabetes diagnosis regardless of whether insulin resistance is present and regardless of whether acromegalic features are visible.Veterinary Clin…+1 IGF-1 reflects growth hormone (GH) secretion over the preceding 24 hours and is preferred over direct GH measurement because of its non-pulsatile release and lesser influence from exercise, stress, and food; direct GH measurement is not commercially available.Journal of Feli… IGF-1 screening should be performed after approximately 6 weeks of exogenous insulin therapy, as insulin deficiency in recently diagnosed cats can suppress IGF-1 readings, producing false negatives.Journal of Feli…+1 Conversely, prolonged insulin treatment beyond 14 months can elevate IGF-1 independently of GH excess, complicating interpretation and requiring additional confirmatory testing.Journal of Feli… Some HST-affected cats will have only a marginal IGF-1 increase, so a borderline result does not exclude the diagnosis.Journal of Feli…

Definitive diagnosis of HST requires elevated IGF-1 confirmed by advanced imaging — computed tomography (CT) or magnetic resonance imaging — demonstrating pituitary enlargement.Journal of Feli… For HC, the low-dose dexamethasone suppression test is preferred in cats and requires a higher dexamethasone dose than used in dogs (0.1 mg/kg); the ACTH stimulation test is more specific but less sensitive.AAHA Clinical G… Endocrine testing for HC should not be performed until diabetic regulation has been attempted for approximately one month, as unregulated diabetes can produce false-positive results.AAHA Clinical G…

Hyperthyroidism is an additional endocrine cause of insulin resistance in cats and can be screened with a total T4 at initial diabetes diagnosis.AAHA Clinical G… It is classified alongside HST and HC within the insulin-resistant diabetes category in cats.Journal of Feli…

Would you like to review the treatment options for confirmed HST, including the comparative outcomes of hypophysectomy, radiation therapy, and cabergoline?