Q. My 22-year-old gelding is starting to show signs of PPID (pituitary pars intermedia dysfunction, formerly called equine Cushing’s disease). I’ve heard that certain seasons can impact testing for this disease. When is the best time of year to test my horse for PPID, and is there a specific type of test I should discuss with my veterinarian?
A. You can test your horse any time of year for PPID, although season does impact test selection and interpretation of results.
Commonly used diagnostic tests for PPID involve measuring resting levels of the hormone adrenocorticotropin (ACTH) or evaluating response of ACTH to thyrotropin-releasing hormone (TRH) stimulation. To measure adrenocorticotropin, your veterinarian will collect a single blood sample. With the TRH stimulation test, your veterinarian will administer a standard dose of TRH intravenously and collect blood for ACTH measurement before and 10 minutes after TRH administration.
The primary factors that influence test selection are season and severity and number of clinical signs. Because the TRH stimulation test looks at hormonal response to a stimulus (i.e., it’s a dynamic test), it is generally more sensitive for identifying PPID. Thus, it is typically selected for horses with early or mild signs of PPID. However, it is not always useful, as during the fall months all horses (both with and without PPID) have more variable responses, making it challenging to interpret the test’s results.
The current recommendation is to measure resting ACTH concentrations in the fall and response to TRH simulation in the non-fall. Seasonal references ranges should be used when interpreting the results of either test. If your horse has advanced signs of PPID, your veterinarian may decide to measure ACTH concentrations in the non-fall. Horses with advanced signs of PPID generally have higher ACTH concentrations, and an ACTH measurement might be more useful when examining changes over time with treatment.
Because it is a dynamic test, the TRH stimulation test can be more variable day to day and less reliable for follow-up. With either test, it is important to use the same laboratory for any follow-up samples, as different laboratories could yield different results.
In addition to the season and presence of PPID, other factors can also impact testing results, including breed, color, thriftiness, feeding practices, and concurrent stress or disease. Grain should be withheld prior to testing. Ideally, testing should be performed on the farm to eliminate the stress of hospitalization.