PPID and ID are two separate equine endocrine diseases, but researchers estimate a 30% comorbidity rate in horses, meaning they can have both conditions. PPID is characterized by muscle atrophy and a curly coat, while ID is more likely to present as obesity and regional fat deposits.

Diagnosing ID in horses involves caution against relying on resting insulin levels, as they can fluctuate. The oral sugar test (OST) is now considered a better alternative for diagnosing ID, involving withholding grain, administering corn syrup, and checking blood insulin levels after 60 minutes.

For diagnosing PPID in horses, the standard of measuring resting ACTH levels can be skewed by various factors. The thyrotropin-releasing hormone (TRH) stimulation test is now recommended for diagnosing PPID, involving drawing blood, injecting TRH, and checking blood results. If a horse is suspected of having both PPID and ID, the same tests are used, with the TRH test recommended before the OST to avoid false positives.