Hi, Lydia - Yes, indeedy, there is a compilation of papers addressing that very thing!  Here is the link to the folders; and below that are a couple of excerpts from the files.

https://groups.yahoo.com/neo/groups/EquineCushings/files/Blood%20Testing%20for%20IR%20%26%20Cushings%20Disease/ACTH%20TESTING%20%26%20INFORMATION/

Scroll to the following files:

ACTH - Valid Biomarker for PPID
ACTH Testing - Support.txt
Blood Tests -- ACTH Rational

And, newer in date:



https://groups.yahoo.com/neo/groups/EquineCushings/files/Blood%20Testing%20for%20IR%20%26%20Cushings%20Disease/

Scroll to the following file: 

ACTH vs Dex Suppression.pdf



And excerpts from two of the studies in the above file:

•••••••• Equine Vet J. 2011 Aug 18. doi: 10.1111/j.2042-3306.2011.00444.x. [Epub ahead of print]
 Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal horses and ponies, and those with pituitary pars intermedia dysfunction.
 Copas VE, Durham AE. Source The Liphook Equine Hospital, Hampshire, UK.
 Abstract
Reasons for performing study: Pituitary pars intermedia dysfunction (PPID) is a common endocrinopathy, frequently diagnosed via plasma adrenocorticotropic hormone (ACTH) concentrations. Seasonal variation in plasma ACTH concentrations has been described in normal horses prompting caution in diagnosing PPID at certain times of the year. The aims of this study were to determine appropriate reference intervals for equine plasma ACTH throughout the year; and to examine the circannual variation of plasma ACTH concentrations in PPID cases. Hypothesis: Plasma ACTH can be used as a test for PPID throughout the year with the use of appropriate reference intervals.
 Methods:
Data for reference interval calculations were obtained from samples collected from inpatients of Liphook Equine Hospital (nonPPID group, n = 156). Data from PPID cases (n = 941) were obtained from samples submitted to the Liphook Equine Hospital Laboratory from horses with a clinical suspicion of PPID found to have plasma ACTH concentrations greater than our upper reference interval for that time of year.
 Results:
 Upper limits for reference interval of plasma ACTH were 29 pg/ml between November and July and 47 pg/ml between August and October. Circannual variation in plasma ACTH occurred in both non-PPID and PPID horses with the highest ACTH concentrations found between August and October in both groups (P<0.0001). The greatest difference between the 2 populations also occurred between August and October.
 Conclusions:
 Plasma ACTH can be used for the diagnosis and monitoring of PPID throughout the year with the use of appropriate reference intervals. These findings demonstrate an increase in pituitary gland secretory activity during the late summer and autumn in both normal and PPID cases. © 2011 EVJ Ltd.
© 2011 The ECIR Group                     



Vet J. 2010 Jul;185(1):58-61.
 The use of adrenocorticotrophic hormone as a potential biomarker of pituitary pars intermedia dysfunction in horses.
  Lee ZY, Zylstra R, Haritou SJ. Source Pegasus Equine Diagnostics Ltd., BioCity Nottingham, Nottingham NG1 1GF, UK.
 Abstract
 Elevated concentrations of plasma adrenocorticotrophic hormone (ACTH) in horses with pituitary disturbances are often associated with dysfunction of the pituitary's pars intermedia. The majority of such animals exhibit an increased susceptibility to laminitis, particularly during the autumn. The 24h plasma ACTH profiles of horses with pituitary pars intermedia dysfunction (PPID), and those of matched controls, were determined in March, June, September and December. Differences in ACTH concentrations between the groups were significant (P<0.0001), regardless of photoperiod, and persisted throughout the 24h cycle in all seasons. ACTH concentrations were significantly higher in September and December than in March and June in both groups of animals. Cosinor analysis of the data indicated that the amplitude of changes was season-independent but higher (P=0.0441) in PPID horses and that a significant interaction (P=0.0096) existed between body condition (control versus PPID) and season of peak ACTH response. There were significant, but opposite, phase-shift differences in June and September. These results suggest that ACTH has a role as a biomarker of equine PPID providing that appropriate 'cut-off' values are used at different times of the year. Copyright 2010. Published by Elsevier Ltd.


J Am Vet Med Assoc. 2004 Apr 1;224(7):1123-7.
 Evaluation of suspected pituitary pars intermedia dysfunction in horses with laminitis.
Donaldson MT, Jorgensen AJ, Beech J. Source Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348, USA.
 Abstract
OBJECTIVE:
To determine prevalence and clinical features of pituitary pars intermedia dysfunction (PPID) in horses with laminitis.
 DESIGN:
 Case series. ANIMALS: 40 horses with laminitis.
 PROCEDURES:
Horses with laminitis that survived an initial episode of pain and were not receiving medications known to alter the hypothalamic-pituitary-adrenal axis were tested for PPID by evaluation of endogenous plasma ACTH concentration. Signalment, suspected cause, month of onset and duration of laminitis, Obel grade of lameness, pedal bone rotation, physical examination findings, results of endocrine function tests, treatment, outcome, and postmortem examination findings were recorded.
RESULTS:
 Prevalence of PPID as defined by a single high plasma ACTH concentration was 70%. Median age of horses suspected of having PPID (n = 28) was 15.5 years, and median age of horses without PPID (12) was 14.5 years. Laminitis occurred most frequently in horses with and without suspected PPID during September and May, respectively. Chronic laminitis was significantly more common in horses suspected of having PPID. In horses suspected of having PPID, the most common physical examination findings included abnormal body fat distribution, bulging supraorbital fossae, and hirsutism. Five horses suspected of having PPID had no clinical abnormalities other than laminitis. Seventeen horses suspected of having PPID that were treated with pergolide survived, and 3 horses that were not treated survived.
 CONCLUSIONS AND CLINICAL RELEVANCE:
Evidence of PPID is common among horses with laminitis in a primary-care ambulatory setting. Horses with laminitis may have PPID without other clinical signs commonly associated with the disease.




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