Equis ISSN 2398-2977

Endocrine: ACTH stimulation test for PPID

Synonym(s): Adrenocorticotrophic hormone stimulation test

Contributor(s): Kathleen P Freeman, Ramiro Toribio, Han van der Kolk, David Rendle


  • Dynamic response of adrenal gland to ACTH stimulation.
  • Reported to result in an exaggerated response in 75% of horses with pituitary pars intermedia dysfunction (PPID) Pituitary pars intermedia dysfunction (PPID), yet only around 20% of horses with PPID will have adrenal pathology.
  • Overlap of values with normal horses.
  • Not considered reliable and no longer used in the diagnosis of PPID.
  • Measures cortisol concentration.


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  • Radioimmunoassay.
  • Chemiluminescence.
  • Method determines required specimen type (plasma or serum).


  • Most veterinary commercial, reference or university laboratories.



  • Check with laboratory to determine that their method has been validated for horses and that reference interval/guidelines for interpretation are available.
  • Estimated sensitivity of 75-80 %.
  • Some overlap with responses obtained in normal horses.


  • No figures available.

Technique (intrinsic) limitations

  • Some overlap of values with responses obtained in normal horses.
  • Resting cortisol (pre-ACTH) may be low, within normal limits or elevated.
  • Both false negative and false positive tests may occur.

Technician (extrinsic) limitations

  • Requires interpretation in conjunction with clinical signs and results of other testing.

Result Data

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Couetil L et al (1996) Plasma adrenocorticotropin concentration in healthy horses and in horses with clinical signs of hyperadrenocorticism. J Vet Intern Med 10 (1), 1-6 PubMed.
  • Van der Kolk J H et al (1995) Laboratory diagnosis of equine pituitary pars intermedia adenoma. Domestic Animal Endocrinol 12 (1), 35-39 PubMed.
  • Sojka J E et al (1995) Evaluation of endocrine function. Vet Clin North Am Equine Pract 11 (1), 415-435 PubMed.
  • Dybal N O et al (1994) Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA 204, 627-632 PubMed.
  • Beech J et al (1987) Evaluation of thyroid, adrenal and pituitary function. Vet Clin North Am Equine Pract 3, 649-661 PubMed.

Other sources of information

  • Reed S M (2004) Pituitary Adenomas; Equine Cushing's Disease. In: Equine Internal Medicine. 2nd edn. Eds: Eds: Reed S M, Bayly W M & Sellon D C. Philadelphia: W B Saunders Co. pp 1327-1330. ISBN-10: 0721697771; ISBN-13: 978-0721697772.
  • Smith B P (2001) Endocrine Disorders. In: Large Animal Internal Medicine. 3rd edn. Mosby, St Louis. pp 1223-1236. ISBN-10: 0323009468; ISBN-13: 978-0323009461.