A blood test to measure the level of circulating adrenocorticotropin hormone or ‘ACTH’ is often used to diagnose PPID (Cushing's Syndrome). However new research published in collaboration with SPILLERS is the first to show that diet can influence ACTH levels and may therefore lead to an incorrect diagnosis. Here we explain more about this research and what it may mean for interpreting ACTH results in the future…

Using ACTH to diagnose PPID

A simple ‘baseline’ ACTH test is often used to diagnose PPID, with a result above a certain level deemed as ‘positive’. However diagnosis is not clear cut as there is ‘grey area’ between the threshold for ‘normal’ and positive results. Diagnosis can also be challenging because:


  • ACTH levels are affected by the season and peak in the autumn
  • Senior horses have naturally higher levels of circulating ACTH compared to adult horses
  • ACTH levels can be affected by stress, exercise and whether or not the horse/ pony has been fasted prior to testing.

When results lie within the ‘grey area’, a more dynamic ‘TRH stimulation test’ which also measures ACTH, is sometimes used to confirm the diagnosis.


The study

Eight healthy adult horses and 8 healthy senior horses were each fed the following 4 diets for 7 weeks:

  • Hay plus a low starch and low sugar feed
  • Hay plus a high sugar feed
  • Hay plus a high starch feed
  • Hay plus a high fibre feed

The ‘TRH test’ was used to measure ACTH levels in the blood at several time points.


Fussy feeder

Key results

  • Senior horses had a much higher level of circulating ACTH when fed the high starch feed compared to adult horses at the same time of year.
  • As expected, ACTH levels were higher in senior horses compared to adult horses.
  • As expected, ACTH levels were higher in October compared to March, May and August regardless of age.

What does this mean?

High starch feeds may increase the level of ACTH in the blood, meaning that some horses may be incorrectly diagnosed with PPID. Vets and laboratories already use seasonally adjusted threshold levels for interpreting ACTH results but the results of this research suggest that diet may also need to be considered.

For more advice on feeding horses and ponies with PPID contact the SPILLERS Care-Line




Jacob S.I., Geor R.J, Weber P.S.D., Harris P.A., McCue M.E. 2018 Effect of dietary carbohydrates and time of year on adrenocorticotropic hormone (ACTH) and cortisol concentrations in adult and aged horses. Domestic Animal Endocrinology, 63, 15-22