Minutes Matter for Horses Undergoing Testing for PPID

You may think you’re a pro at spotting horses with pituitary pars intermedia dysfunction (PPID, Cushing’s) due to their long, thick coat, pendulous abdomen, and muscle loss. What if you wanted to diagnose a horse earlier in the course of disease, before these physical changes occurred? In these cases, the thyrotropin releasing hormone (TRH) test could be the best choice. However, according to experts, it must be accomplished exactingly to yield useful results.^*

In theory, the TRH test is relatively simple to perform. Veterinarians collect a blood sample immediately prior to injecting 1 mg of synthetic TRH directly into the horse’s jugular vein. Ten minutes later, a second blood sample is taken. Adrenocorticotropic hormone (ACTH) levels are then measured in both blood samples and compared. If ACTH levels increase excessively after TRH administration, the horse is presumed to have PPID.

When injected, TRH stimulates an exaggerated ACTH release in horses with PPID due to abnormal tissue in the pituitary gland. Circulating ACTH levels increase rapidly—within 2 to 30 minutes—then decrease just as quickly, providing only a brief window in which to measure elevated ACTH levels and to differentiate normal horses from those with PPID.

As simple as this seems, barriers may preclude the collection of the second sample at the 10-minute mark. For example, veterinarians and their assistants are often completing other tasks during a farm call or a miscommunication may occur between the veterinarian and the timer. Yet, when it comes to this test, inexact timing can affect its precision.

Nicholas Frank, D.V.M., Dipl. A.C.V.I.M., from Tufts Cummings School of Veterinary Medicine, and colleagues demonstrated this fact by performing the TRH test on 24 healthy adult horses. The second sample was collected at the recommended 10-minute mark, as well as at nine and 11 minutes after administering TRH.

When the second sample was collected one minute early or late, 75% of horses had readings that differed by more than 10% of the 10-minute reading.

From a clinical standpoint, this small variation would have resulted in 21% of the tested horses having a different interpretation of the testing result, prompting misdiagnosis.

“Conducting this test properly will help diagnose PPID earlier in the course of disease, allowing initiation of treatment prior to the development of disease sequelae such as laminitis, recurring hoof abscesses, other chronic infections, and behavioral changes,” advised Catherine Whitehouse, M.S., a Kentucky Equine Research nutrition advisor.

In addition to pergolide, the recommended medication for treating PPID, nutritionists typically recommend nutritional management.

“Diets for PPID horses should be based on an appropriate forage source that is easy to chew, as horses diagnosed with PPID are often aged with dental issues,” suggested Whitehouse.

She added, “Assessing body condition score will determine if concentrates are needed to maintain optimal weight and condition. These concentrates should be selected carefully to provide appropriate energy sources, considering the potential presence of insulin dysregulation. Feeds that are high in fiber with low to moderate starch and fat are preferred if a concentrate is needed. For easy keepers or overweight horses, a low-intake protein, vitamin, and mineral supplement is ideal.”

In terms of nutritional supplements, Whitehouse recommends owners of PPID horses consider antioxidants to help combat oxidative stress and low-grade inflammation associated with disease.

 ^*Thane, K., C. Uricchio, and N. Frank. 2022. Effect of early or late blood sampling on thyrotropin releasing hormone stimulation test results in horses. Journal of Veterinary Internal Medicine 16362.