PPID, ID, or Both? Diagnosing Equine Endocrine Disorders

While pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing’s disease) and insulin dysregulation (ID, similar to human prediabetes) are two separate equine endocrine disorders, researchers estimate a 30% average comorbidity rate, meaning horses have both. Erica Macon, MS, PAS, PhD, assistant professor of equine science at Texas A&M University, in College Station, outlined the process of testing a horse for both conditions at the 2024 American Association of Equine Practitioners Convention, held Dec. 7-11, in Orlando, Florida.
What Do PPID and ID Look Like?
The phenotypes, or observable characteristics, of PPID and ID are distinct from one another, Macon said:
- For PPID, veterinarians and horse owners most often notice muscle atrophy (wasting), hypertrichosis (a long, curly coat), patchy shedding, and retained hairs.
- As for ID, they are more likely to note generalized obesity, regional fat deposits (i.e., cresty neck), and chronic or subclinical laminitis as visible—but not pathognomonic (exclusive)—indicators of the condition.
“Appearances can be deceiving, as both lean and fat horses can have neither, either, or both conditions,” Macon explained. “I have seen a morbidly obese—body condition score 9/9 on the Henneke scale—horse with regional fat deposits with insulin levels in the healthy range.” While PPID mainly affects horses 15 or older, the younger equine population is not immune. As for ID, horses of any age, sex, or breed can be affected, although overweight horses and certain breeds (such as Warmbloods and pony breeds) are predisposed.
Equine Endocrine Disorders: Diagnostic Testing for ID in Horses
Macon cautioned horse owners and practitioners against relying on resting insulin levels to diagnose ID because this number can fluctuate based on feeding status (such as a recent grain meal) and seasonality. Additionally, some ID-affected horses have normal resting insulin concentrations, creating a false negative result.
Experts now agree the oral sugar test (OST) is a better alternative. Macon reviewed the proper steps to perform this diagnostic screening:
- Withhold grain for five to six hours before testing.
- Administer a 0.15 mL/kg body weight dose of corn syrup (e.g., Karo syrup), which is very rich in sugar. Include an extra 5 mL to account for spillage.
- Draw blood (in a ethylenediaminetetraacetic acid, EDTA tube, which prevents clotting and preserves cells) 60 minutes after administration.
- Check results. Blood insulin levels over 45 µIU/mL indicate ID.
Equine Endocrine Disorders: Diagnostic Testing for PPID in Horses
Veterinarians have long considered measuring resting ACTH levels the standard for diagnosing PPID in horses. Because stress, geographical region, and seasonal variations can skew results, practitioners have instead turned to the thyrotropin-releasing hormone (TRH, also called thyroid-releasing hormone) stimulation test, which Macon detailed:
- Draw blood in an EDTA blood tube.
- Inject TRH. The dose will depend on body weight. Equids weighing less than 250 kg should receive 0.5 mg TRH, and those weighing more than 250 kg should receive 1.0 mg TRH.
- After TRH administration wait 10 minutes and take another blood sample.
- Check results: 100-200npicograms/mL is suspect, and over 200 pg/mL is likely indicative of PPID.
Comorbidities: ID and PPID in Horses
When veterinarians suspect a horse might have both PPID and ID, the tests remain the same as they would for diagnosing either disorder separately. If testing for both endocrine diseases on the same day, Macon suggests performing the TRH test first, followed by the OST. Conducting the OST before the TRH stimulation test could result in a false positive.
Take-Home Message
Diagnosing equine endocrine disorders involves carefully evaluating clinical signs, following test protocols, and considering confounding factors. “Each endocrine case will present and react differently,” Macon said. By keeping these guidelines in mind, veterinarians can accurately diagnose and manage PPID or ID—or both—in their equine patients.

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