Diagnosing equine pituitary pars intermedia dysfunction
In this article our laboratory expert, Stacey A Newton, BVSc, FRCPath, CertEM (Int Med), PhD, MRCVS, explores the diagnosis of equine pituitary pars intermedia dysfunction (PPID), also known as equine Cushing’s disease. PPID is a condition caused by an adenoma of the pituitary pars intermedia, which produces excess secretions of adrenocorticotropic hormone (ACTH). The diagnosis of PPID requires a combination of detailed history, clinical signs, and a combination of diagnostic tests.
The article highlights the importance of considering the effect of season and the individual’s current health status when diagnosing PPID. It also notes that there is no ideal diagnostic test for PPID, and that a combination of tests is often used to avoid false positive and false negative results.
The article discusses several diagnostic tests for PPID, including:
- Basal cortisol: This test is not useful for diagnosing PPID as cortisol levels can be normal or low even in horses with PPID.
- Insulin: Elevated insulin levels can be seen in horses with PPID, but insulin levels can also be elevated in horses with other conditions such as obesity and equine metabolic syndrome.
- Endogenous ACTH: This test is considered the most reliable test for PPID and measures the production of ACTH by the pituitary gland. However, ACTH levels can be affected by stress, illness, exercise, and diet.
- TRH stimulation test: This test measures the response of ACTH to thyrotropin-releasing hormone (TRH). It is considered more accurate than basal ACTH levels alone.
- Overnight dexamethasone suppression test: This test is not recommended for diagnosing PPID in the autumn months due to increased false positive results.
- TRH stimulation test: This test measures the response of ACTH to TRH and is considered more accurate than basal ACTH levels alone.
- Urine cortisol:creatinine ratio: This test is a sensitive screening test for excluding PPID, but it should not be used to make a diagnosis.
The article concludes that a diagnosis of PPID requires a combination of detailed history, clinical signs, and a combination of diagnostic tests. The effect of season and the individual’s current health status should also be taken into account.
