Equis ISSN 2398-2977

Endocrine: TRH stimulation test - hyperadrenocorticism

Synonym(s): Thyrotropin-releasing hormone TRH stimulation test

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


  • Stimulation test using thyrotropic releasing hormone (TRH).
  • Greater increase in blood cortisol following administration of TRH in Cushingoid horses.
  • Described for use in assessment of thyroid function and for diagnosis of pituitary adenoma (equine Cushing's disease)    Pituitary pars intermedia dysfunction (PPID).
  • Most commonly used for diagnosis of pituitary adenoma - dynamic response of pituitary gland in response to TRH stimulation results in release of ACTH (or corticotropin-like intermediate lobe peptide CLIP) with exaggerated increase in cortisol levels in some horses with pituitary adenoma   Pituitary pars intermedia dysfunction (PPID).


This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login



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



  • Check with laboratory to determine that their method has been validated for horses and that reference interval/guidelines for interpretation are available.

Technique (intrinsic) limitations

  • Some overlap of values with responses obtained in normal horses.
  • Both false negative and false positive tests may occur.
  • If resting cortisol level is within normal limits or elevated in a horse with pituitary adenoma   Pituitary pars intermedia dysfunction (PPID)  a single TRH stimulation test may not result in a detectable increase in cortisol at 15 min post-TRH   Protirelin  administration (false negative); in these cases cosider baseline ACTH concentrations or the dexamethasone suppression test.

Technician (extrinsic) limitations

  • Resting cortisol (pre-TRH) may be low, within normal limits or elevated.
  • Requires interpretation in conjunction with clinical signs and results of other testing.

Result Data

This article is available in full to registered subscribers

Sign up now to purchase a 30 day trial, or Login

Further Reading


Refereed papers

  • Recent references fromPubMedpublished during the last 12 months.
  • Frank N et al(2006)Evaluation of the combined dexamethasone suppression/thyrotropin-releasing hormone stimulation test for detection of pars intermedia pituitary adenomas in horses. J Vet Intern Med20(4), 987-993PubMed.
  • van der Kolk J H et al(1995)Laboratory diagnosis of equine pituitary pars intermedia adenoma. Domestic Animal Endocrinol12(1), 35-39PubMed.
  • Thompson J C et al(1995)Problems in the diagnosis of pituitary adenoma (Cushing's syndrome) in horses. New Zealand Vet J43(2), 79-82PubMed.
  • Sojka J E et al(1995)Evaluation of endocrine function. Vet Clin North Am Equine Pract11(1), 415-435PubMed.
  • Dybdal N O et al(1994)Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA204, 627-632PubMed.
  • Beech J et al(1987)Evaluation of thyroid, adrenal and pituitary function. Vet Clin North Am Equine Pract3, 649-661PubMed.
  • Beech J et al(1985)Hormonal response to thyrotropin-releasing hormone in healthy horses and in horses with pituitary adenoma. Am J Vet Res46, 1941-1943PubMed.

Other sources of information

  • Toribio R E (2004)Pars Intermedia Dysfunction (Equine Cushing's Disease).In: Equine Internal Medicine.Eds: Reed S M, Bayly W M & Sellon D C. 2nd edn. Saunders, St Louis. pp 1327-1339.
  • Reed S M (1998)Pituitary Adenomas; Equine Cushing's Disease.In: Equine Internal Medicine. Eds: S M Reed & W M Bayly. Philadelphia: W B Saunders Co. pp 912-916.
  • Dybdal N O (1990)Endocrine Disorders.In: Large Animal Internal Medicine. Ed: B Smith. C V Mosby, St Louis. pp 1296-1302.