Pituitary pars intermedia dysfunction in horses is most often due to hyperplasia or an adenoma of the pars intermedia of the pituitary gland.
Measurement of endogenous ACTH concentrations using seasonal reference intervals has largely superseded the overnight dexamethasone suppression test as the screening test of choice for PPID. However, the overnight dexamethasone suppression test may still be used outside of the autumn period (a high rate of false positives occurs with this test during the autumn), as both tests perform relatively similarly during the non-autumn period. The measurement of endogenous ACTH during the autumn using seasonal reference ranges is likely to give the highest accuracy in diagnosis.
There is seasonal variation in the baseline serum ACTH concentrations in normal horses and ponies. Therefore appropriate seasonal reference intervals must be used in interpretation. Sensitivity is also an issue for both tests and retesting in 3-6 months is recommended if testing in inconclusive despite a suspicious clinical picture.
An ACTH stimulation test has been described for horses but this is not recommended as a diagnostic test as it does not adequately differentiate normal horses from those with PPID.