Pituitary Pars Intermedia Dysfunction in Horses

HAC is seen in older horses and is caused by hyperplasia or adenomas of the pars intermedia of the pituitary and is referred to as pituitary pars intermedia dysfunction (PPID). Pituitary pars intermedia dysfunction is believed to arise due to a primary hypothalamic abnormality causing loss of inhibitory dopaminergic influence from the hypothalamus; this loss of dopamine influence results in hyperplasia of the pars intermedia. Clinical signs are due to local compression of the hypothalamus as well as cortisol excess, and include polyuria, polydipsia, muscle atrophy, excessive sweating (hyperhidrosis), and long hair coat from failure to shed.

The most consistent laboratory findings are fasting hyperglycemia and glucosuria, probably from insulin resistance and chronic excessive food intake. Baseline cortisol is not useful in differentiating normal horses from those with PPID. Although the overnight dexamethasone suppression test (DST) has traditionally been the recommended diagnostic test for PPID, endogenous ACTH testing has become more popular and is useful as long as seasonal effects and normal pulsatile release are considered when interpreting results. A PPID working group (Equine Endocrinology Group, EEG) has established recommendations for the diagnosis and treatment of PPID and these are regularly updated on their website (https://equineendocrinologygroup.org/). This group no longer recommends using the DST as it is no better at detecting PPID than other tests and dexamethasone-induced laminitis is a concern.

In addition to measuring resting ACTH concentrations, the EEG also recommends a thyrotropin-releasing hormone (TRH) stimulation test (see EEG website as above) especially in those horses with early stage PPID or horses suspected to have PPID but with normal resting ACTH concentrations. TRH causes the release of hormones including ACTH from the pituitary gland, therefore the ACTH concentration following TRH administration should be increased in horses with PPID. The TRH stimulation test should only be used between December and June, which is the only period for which RI have been currently established. Since testing for PPID may not be affected equally by season in all geographic locations, further studies are underway to determine if season-specific RI are required for all areas.

Since many of the clinical signs of PPID relate to the tumor mass rather than the effects of hyperglycemia and hypercortisolism, antiadrenocortical medications and insulin are not useful. Dopamine agonists have been reported to correct many of the clinical manifestations of PPID in horses.