Validation of a Protirelin Thyrotropin-Releasing Hormone (TRH) Stimulation Test in Horses


The thyrotropin-releasing hormone (TRH) stimulation test is used to diagnose Cushing’s disease (pituitary pars intermedia dysfunction; PPID) in horses.  It is particularly useful for detecting PPID in the earliest stages before signs become obvious.   Thyrotropin-releasing hormone is naturally produced in the body and is also synthesized by companies for research and diagnostic purposes.  There are two forms currently available – one that has been used for many years to develop the test (but is only available to researchers) and a product that has been used in human medicine called protirelin.  This study compares the two forms of TRH and we hope to establish that protirelin can be used for this test in horses.  It has not been possible for veterinarians to purchase TRH in the past, but protirelin has recently become available.  We will therefore perform one stimulation test with the TRH that we have been using for many years and a second test with protirelin, spaced 7 days apart.

The TRH stimulation test involves fasting your horse and then collecting 3 blood samples (10 mL each) from the jugular vein using a needle before, and 10 and 30 minutes after, injecting TRH in the vein.  Concentrations of a hormone called adrenocorticotropic hormone (ACTH) are measured.

This study involves two visits to your farm or two appointments at the Hospital for Large Animals, 7 days apart.

Inclusion Criteria:

Barns with multiple horses with suspected or confirmed Cushing’s disease.

Horse owners must allow their horse to be tested twice exactly 7 days apart.

Exclusion Criteria:

Horses that are currently suffering from other diseases.  For example, any medical problem causing fever.

Blood samples must be collected without causing significant stress, so horses with a fear of needles are excluded.

Client Benefits:

This study covers the cost of measuring ACTH in the blood samples collected, but horse owners are financially responsible for costs associated with two veterinarian visits to farm (or appointments in the Hospital for Large Animals) and professional time for performing the tests.

Contact Information:

Diane Welsh, Clinical Trial Technician