Pituitary pars intermedia dysfunction occurs commonly in aged horses despite under-recognition by owners. The increased risk of PPID with age supports that this is an ageing associated condition. Aged horses with clinical or historical signs consistent with PPID, especially owner-reported hirsutism (delayed shedding and/or long hair coat), should be tested and appropriate treatment instituted.

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Summary Reasons for performing study: Serum insulin concentration and its use as a prognostic indicator in horses with equine Cushing's syndrome (ECS) have been poorly documented. Objectives: To examine daily insulin variations in horses with ECS and the effect of treatment using trilostane, a competitive inhibitor of 3β‐hydroxysteroid dehydrogenase. Further, we aimed to examine the relationship between baseline serum insulin concentration and survival in horses with ECS. Methods: Basal serum insulin concentrations were measured in 20 confirmed ECS cases by taking blood at regular 4 h intervals for 24 h (1200, 1600, 2000, 2400, 0400 and 0800 h) before treatment (Day 0) and 10 days, and 30 days and 1–2 years after the onset of trilostane therapy. The temporal pattern of insulin was analysed using a linear mixed model approach, and the prognostic value of measurements on Day 0 assessed using receiver‐operating characteristic analysis. Results: Horses with ECS showed a diurnal pattern of serum insulin concentration, highest value at 1200 h, and this pattern was not altered by treatment with trilostane. Furthermore, despite a mild increase of serum insulin concentrations after 10 days of trilostane therapy, insulin concentration was unaffected in the long term. Low serum insulin concentrations at the beginning of the trial were significantly associated with improved survival to 1–2 years. The 1200 h sampling before treatment had the highest prognostic value for prediction of survival with a sensitivity and specificity of at least 90% for serum insulin at <62 and >188 μu/ml to predict survival and nonsurvival, respectively. Conclusions and potential relevance: Insulin is a useful prognostic indicator for ECS, but potentially large variations can occur throughout a 24 h period, indicating a single sample may not be representative. Serum insulin concentration did not increase over 1–2 years in horses receiving trilostane therapy.

Owners demonstrated concern for the health and welfare of their aged horses, but the lack of appropriate recognition of the significance of clinical signs and limited veterinary care suggest a need for increased client communication and veterinarian-client interaction.