Equis ISSN 2398-2977

Endocrine: TRH stimulation test - thyroid function

Synonym(s): TRH stimulation test

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


  • As the hypothalamic-pituitary-thyroid axis forms a dynamic system, stimulation tests are preferable and are usually more diagnostic than single-sample tests.
  • Stimulation test using thyrotropin (TSH) releasing hormone (TRH).
  • Greater increase in blood cortisol following administration of TRH in Cushingoid horses.
  • This protocol will describe its use for evaluation of thyroid function.
  • Dynamic response of pituitary gland and thyroid gland in response to stimulation.
  • Assessing thyroid gland function based on thyroid hormones (T3, T4) concentrations has major limitations as many factors (disease, phenylbutazone, corticosteroids, thyroid hormone supplementation) affect their concentrations (typically low despite normal thyroid gland function).
  • Thus functional tests are important in the diagnosis of hypothyroidism.


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  • Radioimmunoassay.
  • Chemiluminescence.
  • Automated enzyme immunoassay.
  • Enzyme-linked immunosorbant assay (ELISA).
  • Competitive protein-binding assays.
  • Method determines required specimen type (plasma or serum).


  • Most commercial, reference or university laboratories.



  • No data available.
  • Dynamic testing may help differentiate true thyroid dysfunction from abnormal results due to a variety of other endogenous exogenous factors.


  • Help assess the pituitary-thyroid-hypothalamic axis and ability of thyroid gland to respond to stimulation.

Technique (intrinsic) limitations

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

Result Data

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Further Reading


Refereed papers

  • Recent references fromPubMedpublished during the last 12 months.
  • Singh A K et al(1997)Validation of non-radioactive chemiluminescent immunoassay method for analysis of thyroxine and cortisol in blood samples obtained from dogs, cat and horses. J Veterin Diag Invest9(3), 261-268.
  • Allen A L et al(1997)Effects of delayed serum separation and long-term storage on the measurement of thyroid hormones in equine blood samples. Vet Clin Pathol26(1), 10-13PubMed.
  • Sojka J E et al(1995)Evaluation of endocrine function. Vet Clin North Am Equine Pract11(2), 415-435PubMed.
  • Dybdal N O et al(1994)Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA204(2), 627-632PubMed.
  • Duckett W M et al(1989)Thyroid hormone periodicity in healthy adult geldings. Equine Vet J21(2), 125PubMed.
  • Glade M J & Reimers T J (1985)Dietary energy supply on serum thyroxine tri-iodothyronine and insulin concentrations in young horses. J Endocrinol104, 93PubMed.

Other sources of information

  • Toribio R E & Duckett W M (2004)Thyroid Gland. In: Equine Internal Medicine. Eds: Reed S M, Bayly W M, & Sellon D C. 2nd edn. Saunders, St Louis. pp 1340-1356.
  • Duckett W M et al(1998)Thyroid Gland.In: Equine Internal Medicine.Eds: S M Reed S M & W M Bayly. Philadelphia: W B Saunders Co. pp 916-925.
  • Powell D et al(1997)The Effect of Feeding Restricted Diets on T4 and T3 Concentrations in the Exercising Horse. Proc 15th Eq Nutr and Physiol Symposium.Fort Worth, TX. pp 33-38.
  • Allen A L et al(1993)Congenital Gypothyroidism Dysmaturity and Skeletal Lesions in Western Canadian Foals. Proc 39th Ann Conven Am Assoc Eq Pract.San Antonio, TX. pp 207-208.
  • Dybdal N O et al(1990)Endocrine Disorders.In: Large Animal Internal Medicine.Ed: B Smith. C V Mosby, St Louis. pp 1296-1302.