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Endocrine: TRH stimulation test – hyperadrenocorticism
Synonym(s): Thyrotropin-releasing hormone TRH stimulation test
Overview
- 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).
Uses
In combination
- With clinical signs, especially hirsutism
, polyuria/polydipsia Polydipsia / polyuria , hyperhydrosis and laminitis Foot: laminitis for diagnosis of pituitary adenoma Pituitary pars intermedia dysfunction (PPID). - In combination with endogenous ACTH levels.
- In combination with the dexamethasone suppression test Endocrine: dexamethasone suppression test .
Sampling
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Tests
Methodologies
- Radioimmunoassay.
- Chemiluminescence.
- Method determines required specimen type (plasma or serum).
Validity
Sensitivity
- 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
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Further Reading
Publications
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.