Pituitary Pars Intermedia Dysfunction (PPID) remains the most commonly diagnosed endocrinopathy in the horse. Dopaminergic neurons in the hypothalamus degenerate leading to a loss of the normal tonic inhibition of the pars intermedia. This leads to overproduction of many peptides including pro-opio melanocortin, α-melanocyte-stimulating hormone, corticotrophin-like intermediate peptide and adrenocorticotropic hormone (ACTH). Clinical signs of PPID include hirsutism, sweating, abnormal fat deposition, muscle wastage, laminitis, lethargy and polydipsia.
Diagnosis in advanced cases is often easy based on the presence of clinical signs. However, laboratory diagnosis is important for less advanced cases, and to aid in monitoring the response to treatment. Understanding the limitations of the currently available diagnostic tests is also important to avoid overdiagnosis of the disease and unnecessary treatment.
An excellent resource is the Equine Endocrinology Group website.
Sampling protocol:
This is the recommended first line test for older horses in which you have a strong suspicion of clinical disease based on the presence of clinical signs.
The test should be avoided in young horses (less than 10 years old), or those without any clinical signs, as the positive predictive power of the test in these circumstances is low and many false positives occur. Horses with early disease often have normal or equivocal ACTH concentrations, and further testing with a TRH stimulation test, or repeated monitoring may be needed to evaluate further.
Season has a major impact on ACTH concentration in both normal and PPID affected horses and it is important that results are interpreted in line with seasonal reference ranges.
As there is significant variation in ACTH concentrations between individuals, a single positive threshold is not recommended and an equivocal range is reported. Results that fall in this range should be interpreted in conjunction with the clinical signs and age of the horse. Depending on these factors it may be appropriate to perform a TRH stimulation test, monitor and re-test or treat the horse.
The following table summarises the suggested cut-offs using the Tosoh AIA-900. Results are listed in pg/ml.
| Time of Year (Northern Hemisphere) | PPID Unlikely | Equivocal (requires strong clinical signs, retesting or TRH stimulation test to confirm diagnosis) | PPID Likely |
| December – June | <12 | 12-32 | >32 |
| July and November | <16 | 16-44 | >44 |
| August | <22 | 22-52 | >52 |
| September – October | <23 | 23-58 | >58 |
The breed of horse being tested should also be considered. Pony breeds, Arabians and donkeys have all been shown to have higher plasma ACTH concentrations especially in the Autumn months. Concurrent disease, stress and travel can also have an impact on ACTH concentration and the presence or absence of these factors should be considered when interpreting any result.
This is currently the recommended dynamic test for the diagnosis of PPID. The test relies on an excessive pituitary response to the administration of thyrotropin-releasing hormone (TRH) in horses with PPID when compared to normal horses. However, the test still has limitations and ongoing validation and research is needed to help us fully understand how to interpret the results. Usefulness of the test is limited to the period from January to June and updated diagnostic thresholds will be published once established for the Tosoh AIA-900 analyser.
The following table summarises the current diagnostic thresholds using the Immulite analyser. New thresholds are being generated for the Tosoh AIA-900 analyser.
| Time of Year | Negative | Equivocal (requires strong clinical signs, retesting or TRH stimulation test to confirm diagnosis) | PPID Likely |
| January to June | <100 | 100 – 200 | >200 |
| July to December | <100 | TRH stimulation testing can only be used to identify negative cases in these months due to many false positives |
Compounded TRH can be purchased here
The product is sold as Thyrotropic Releasing Hormone (TRH) 1mg/5ml
Side effects are rare but include trembling, lip-smacking and flehmen type behaviour.
Insulin dysregulation is a common concurrent problem in horses with PPID. It is good practice to evaluate this as a minimum by measurement of resting insulin and glucose concentrations or by performing a dynamic sugar challenge test. This can be useful when deciding about the need for pergolide treatment in a horse with equivocal results. The presence of hyperinsulinaemia may be associated with an increased risk of laminitis and consequently a greater need for treatment for PPID.
ACTH and Insulin in Equids – the significance of the analytical method.
References: