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: adenoma  .
  • 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: adenoma  .


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  • 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: adenoma  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

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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.