Diagnosing Pituitary Pars Intermedia Dysfunction(PPID, Equine Cushing’s Disease)
Pituitary Pars Intermedia Dysfunction (PPID) remains the most commonly diagnosed endocrinopathy in the horse. Degeneration of the dopaminergic neurons in the hypothalamus causes a loss of the normal tonic inhibition of the pars intermedia.
This leads to overproduction of many peptides including proopiomelanocortin, α-melanocytestimulating hormone, corticotrophin-like intermediate peptide and 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.
The recommended diagnostic tests for PPID have not changed in the last few years and diagnosis relies mainly on measurement of basal Adrenocorticotropic hormone (ACTH) concentration or the response to the administration of Thyrotropinreleasing hormone (TRH).
Recent research has focused on learning more about the interpretation of these tests under different circumstances. The result of this is that many more ‘grey-areas’ have become apparent and having a clear cut-off value to reach a diagnosis is more difficult. It is likely that over the next few years, we will be able to reach a better understanding of how to most effectively use and interpret these tests.
This document summarises some of the currently available information, which should help in reaching a diagnosis in clinical practice. An excellent resource is the Equine Endocrinology Group website http://sites.tufts.edu/equinee...
Basal plasma ACTH concentration
This is the recommended first line test Sampling protocol:
- Collect a single EDTA blood sample at any time of day • Chill within 3 hours
- Separate the plasma by centrifugation or gravity
- Ship to the laboratory overnight with cool packs (maximum delay 48 hours)
- If there will be a delay between sample collection and analysis then centrifuged plasma can be frozen and is then stable for many weeks. Gravity separated plasma should not be frozen as this will lead to a spurious increase in ACTH concentration
Interpreting the result
When using this test in horses with advanced clinical disease the test has both a high sensitivity and specificity. However, when this test is used as a screening test in horses with a lower incidence of disease (for example younger horses with vague clinical signs) then the positive predictive power of the test falls significantly and many false positives occur.
Season (and specifically day-length) has a major impact on ACTH concentration in normal horses. Healthy horses have a significant increase in ACTH concentration in the Autumn. Recently, much higher cut-off values for the diagnosis of PPID in the Autumn months have been proposed than those that have traditionally been used. There is also much less certainty that testing in the Autumn leads to an increased sensitivity of diagnosis rather than a greater rate of false positive test results.
The following table summarises the suggested cut-offs agreed at the recent ACVIM endocrinology special interest group meeting. Results are listed in pg/ml.
|Time of Year||Negative||Equivocal||Positive|
|Mid-November - Mid-July||<30||30 - 50||>50|
|Mid-July - Mid-November||<50||50 - 100||>100|
Thyrotropin releasing hormone stimulation test (TRHST)
This is currently considered the most accurate 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 research is needed to help us fully understand how to interpret the results. This test is appropriate when the results of a basal ACTH test have been equivocal, or in a case in which PPID is still suspected despite a negative ACTH test result.
- Collect an EDTA sample for baseline measurement of ACTH
- Inject 1mg TRH intravenously
- Collect a second EDTA sample exactly 10 minutes later
- The plasma should be handled as described in the ACTH section
Availability of TRH
Pharmaceutical grade TRH is not available on the veterinary or human markets in the UK.
Chemical grade TRH can be cheaply purchased from https:// p h o e n i x p e p t i d e . c o m / o rd e r _ information_europe, the specific product is listed at https://www. phoenixpeptide.com/products/view/Peptides/062-10
We believe this to be legal via the cascade but full VMD guidance can be found at https://www.gov.uk/ guidance/the-cascade-prescribingunauthorised-medicines
If you need any help ordering TRH or would like us to supply a small quantity of the product please contact Kathryn Thornton on 01638 577754.
Interpreting the result
A cut-off of 100pg/ml was initially used to differentiate between healthy horses and those with PPID. Seasonal differences in the response to TRH occur in healthy horses. This has led to the recommendation that this test is best avoided in the Autumn months.
However, recent work has further evaluated this and this in conjunction with a consensus from the ACVIM special interest endocrinology group has led to the following recommendations. Results are listed in pg/ml.
|Time of year||Negative||Equivocal||Positive|
|December - June||<110||110 - 200||>200|
|July and November||<110||110 - 250||>250|
|August / September / October||<110||110 - 500||>500|
These guidelines are obviously going to lead to a far greater number of equivocal results. When this happens a decision will have to be made on an individual case basis. In horses with equivocal results that have clinical signs highly suggestive of PPID, or active laminitis, it may be prudent to treat the horse with pergolide. In other situations, it will be more appropriate to wait and retest the horse in 2 – 3 months.
Side effects are rare but include trembling, lip-smacking and flehmen type behaviour.
Dexamethasone suppression test
This test was previously considered the gold standard for the diagnosis of PPID. However, this test is much less reliable than the other tests previously discussed. The test is also unreliable in the Autumn months and requires the administration of corticosteroids which is often contra-indicated in horses judged to be at high risk for the development of laminitis. Consequently, this test is not currently recommended.
Other tests in horses with PPID
Insulin dysregulation is a common concurrent problem in horses with PPID. It is good practice to evaluate this as a minimum by measurement of basal insulin and glucose concentrations or by performing a dynamic sugar challenge test (link to insulin document). Performing a TRHST after an oral sugar test has been shown to slightly reduce the ACTH response and hence it may be preferable to perform these tests on different days.
- Adams A. Evaluating seasonal influences on hormone responses to a diagnostic test advocated for early diagnosis of pituitary pars intermedia dysfunction. 2017.
- Havemeyer International Endocrine Symposium, Miami
- Beech J, Boston R, Lindborg S, Russell GE. Adrenocorticotropin concentration following administration of thyrotropin-releasing hormone in healthy horses and those with pituitary pars intermedia dysfunction and pituitary gland hyperplasia. J Am Vet Med Assoc. 2007; 231:417-26
- Beech J, Boston R, Lindborg S. Comparison of Cortisol and ACTH Responses after Administration of Thyrotropin Releasing Hormone in Normal Horses and Those with Pituitary Pars Intermedia Dysfunction. J Vet Intern Med. 2011; 25:1431-8
- Copas VE, Durham AE. (2012) Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal horses and ponies, and those with pituitary pars intermedia dysfunction. Equine Vet J. 2012 Jul;44(4):440–3
- Diez de Castro E, Lopez I, Cortes B, Pineda C, Garfia B, Aguilera-Tejero E.
- Influence of feeding status, time of the day, and season on baseline adrenocorticotropic hormone and the response to thyrotropin releasing hormone-stimulation test in healthy horses. Domest Anim Endocrinol. 2014 Jul;48:77-83.
- Funk RA, Stewart AJ, Wooldridge AA, Kwessi E, Kemppainen RJ, Behrend EN, Zhong Q, Johnson AK. Seasonal changes in plasma adrenocorticotropic hormone and α-melanocyte-stimulating hormone in response to thyrotropin-releasing hormone in normal, aged horses. J Vet Intern Med. 2011 May-Jun;25(3):579-85.
- Goodale L, Frank N, Hermida P, D’Oench S. Evaluation of a thyrotropin-releasing hormone solution stored at room temperature for pituitary pars intermedia dysfunction testing in horses. Am J Vet Res. 2015; 76(5):437-44
- McFarlane D, Maxwell LK. Establishment of a reference interval for plasma ACTH concentration in aged horses. 2017. Havemeyer International Endocrine
- Symposium, Miami
- Rendle DR, Laboratory diagnosis of the endocrine causes of laminitis Livestock July/August 2017, Volume 22 No 4
- Restifo MM, Frank N, Hermida P, Sanchez-Londoño A. Effects of withholding feed on thyrotropin-releasing hormone stimulation test results and effects of combined testing on oral sugar