If you suspect that your horse has Cushing’s disease, avoid having him tested during the autumn months or you may receive false-positive results.

Researchers at the University of Pennsylvania have found that seasonal variations in adrenocorticotropic hormone (ACTH) can affect the results of the dexamethasone-suppression test, considered the “gold standard” for diagnosing Cushing’s.

The test works by introducing dexamethasone, a potent synthetic form of the hormone cortisol, into the horse’s system and then testing his blood: If he is healthy, his pituitary gland will respond to the dexamethasone by stopping ACTH production, and blood levels of cortisol will be normal. But if the horse has Cushing’s, his malfunctioning pituitary will continue producing ACTH, which causes the adrenal gland to produce cortisol unabated, and his blood will contain elevated levels of the hormone.

However, Mark Donaldson, VMD, recently discovered that another factor–the time of year–also seems to influence equine ACTH blood levels. “In September I took samples from a group of horses who had no signs of Cushing’s. To my surprise, most of the tests showed elevated ACTH levels,” says Donaldson. “I thought it was a laboratory error, so I retested the same horses again in January using the same lab and they all came back as having normal ACTH levels.”

To investigate further, the researchers used 39 horses with ages ranging from 2 to 26. Blood ACTH levels were recorded for each horse in September 2002 and January, May and September 2003. Dexamethasone-suppression tests were performed in January and September 2003.

“We found that while nearly all the horses had ACTH levels within normal ranges in January and May, most had what would be considered abnormally elevated levels in September,” reports Donaldson. Specifically, only 5 and 8 percent of the horses had ACTH levels in the normal range in September 2002 and 2003, respectively.

This indicates “a natural seasonal fluctuation in ACTH that directly affects test results,” says Donaldson, adding that it’s impossible to pinpoint why these variations occur, but they could be a response to the shortening photoperiod or part of the process for storing body fat for the winter ahead.

Donaldson recommends postponing the testing of horses for Cushing’s until after the fall whenever possible. “If a horse is obviously in crisis–with strong clinical signs and at risk of laminitis–of course, you need to treat him. But if his signs are so mild that you’re merely suspicious of Cushing’s, wait a few months before you get a diagnosis.”

This article originally appeared in the September 2005 issue of EQUUS magazine.