Chilled EDTA plasma
Normally secreted by corticotrope cells in the pars distalis although, in PPID, melanotrope cells from the pars intermedia secrete large quantities. Normal (pars distalis-derived) ACTH stimulates glucocorticoid secretion from the adrenal cortex, although ACTH from the pars intermedia is significantly less bioactive leading to typically normal cortisol levels in PPID cases.
ACTH might sometimes increase mildly to moderately in response to stress/pain/disease but the highest measured concentrations are associated with PPID.
ACTH is a very sensitive test for PPID but might lack specificity (i.e. normal horses tend to have normal concentrations, although not all PPID cases have high concentrations).
ACTH demonstrates significant seasonal changes in concentration with the highest concentrations in late September and the lowest in April. Different breeds also demonstrate different levels of ACTH, especially in the autumn, with donkeys, Shetland ponies, Welsh ponies and Arabians tending to have higher values, and Thoroughbreds, Warmbloods and Connemara ponies tending to have lower values.
Measuring ACTH following exogenous TRH administration may increase the accuracy of testing for PPID.