Lead a veterinarian to a horse with equine Cushing's disease and he or she will likely try to diagnose the horse by measuring blood levels of adrenocorticotrophic hormone (ACTH) in a single sample at a single point in time. Lead researchers from Pegasus Equine Diagnostics Ltd., in Nottingham, United Kingdom to the same horse and they'll measure ACTH levels over 24 hours at multiple times of the year and claim that ACTH is a biomarker for the disease.

Equine Cushing's disease (pituitary pars intermedia dysfunction, PPID) is a relatively common hormonal disturbance in older horses and ponies. Most affected horses have increased blood levels of ACTH, but because of normal, cyclical rhythms in the secretion of ACTH, blood levels can vary markedly over a 24-hour period and depending on the season. This makes diagnosing Cushing's disease challenging when only a single ACTH level is measured.

"While it is possible to diagnose equine Cushing's disease based on the ACTH concentration in a single blood sample, improper sample handling, stress, pain, and other factors can all affect the test results," relayed the researchers. "This is why reservations have been raised by experts in this field regarding the value of using ACTH to diagnose this disease."

Convinced of the merits of ACTH testing, the research team measured ACTH levels in healthy older horses over a 24-hour period and compared them to the ACTH levels in horses with Cushing's disease. ACTH was measured in blood samples collected in March, June, September, and December