Liphook Equine Hospital is at the forefront of research in this area, having initially suggested that a two-tier reference interval should be used, with a cut-off of 47 pg/ml from August to October, and a cut-off of 29 pg/ml the rest of the year.
However, this was still a rather crude measure, not allowing for the gradual increase and decrease in ACTH through the seasonal rise, and new weekly cutoff values are now being used at Liphook, based on the ACTH results of over 35,000 horses, with ACTH concentrations increasing shortly after the June solstice (when day length starts to decrease), peaking around the September equinox, and declining back to basal levels by the end of November.
Interestingly, Liphook found that insulin and glucose concentrations did not appear to increase alongside ACTH, and if anything insulin and glucose tended to be higher from December to March.
What isn't clear from these new cutoffs though is how the transitional zone around them should be interpreted, as research by David Rendle et al published in 2013 suggested "Our results support previous assertions that ACTH is a reliable means of differentiating non-PPID and PPID horses but indicate that there is a transitional zone, around currently recommended cut-offs for diagnosing the disease, at which further testing should be considered."
Rendle DI, Litchfield E, Heller J, Hughes K J
Investigation of rhythms of secretion and repeatability of plasma adrenocorticotropic hormone concentrations in healthy horses and horses with pituitary pars intermedia dysfunction
Equine Veterinary Journal Vol 46, Issue 1, pages 113–117, January 2014 (published online Aug 2013)
Seasonal Changes in ACTH Secretion - Liphook Equine Hospital
Further Observations of Seasonality of Pars Intermedia Secretory Function in 30,000 Horses and Ponies - Andy Durham - p 11-12 Dorothy Russell Havemeyer Foundation Equine Geriatric Workshop II 3rd Equine Endocrine Summit 2014