Felis ISSN 2398-2950

Thyroid: TRH stimulation test

Synonym(s): thyrotropin releasing hormone stimulation test

Contributor(s): Carmel Mooney, Mark Peterson, Roger Powell

Overview

  • Thyrotropin releasing hormone (TRH, from the hypothalamus) stimulates production of TSH from the anterior pituitary to regulate the synthesis and secretion of thyroxine (T4) and tri-iodothyronine (T3) . This is a negative feedback system, the released (free) T4 and T3 inhibiting further release of TSH (and possibly TRH). 
  • Using and mimicking this dynamic hypothalamic-pituitary-thyroid axis by exogenous stimulation (eg injecting exogenous TRH) tries to highlight excessive and often independent production of T4 (and T3) by the thyroid gland.  
  • Thyroxine (total thyroxine/tT4/T4) is the main secretory product of thyroid gland, as both free and protein bound forms. Over 99% of thyroxine is reversibly bound to carrier proteins, their nature though less clear in cats (cf. dogs). 
  • Such testing can be more sensitive than single hormonal measurements. TRH stimulation though here relies on its TSH effect, which is then indirectly measured by relative T4 (cf. measurements of TSH as may be done in humans). Current veterinary assays, especially TSH, are not reliable enough for such testing though. 
  • It is also slightly counter-intuitive to confirm ‘hyper-production’ and clinical hyperthyroidism by further stimulating the thyroid gland (cf. trying to suppress it). This test relies upon the axis becoming ‘exhausted’ or poorly responsive from long term increases in T4, so TRH stimulation no longer has any effect - the pituitary no longer releases TSH, so the thyroid gland does not then release more thyroxine either. However, euthyroid axes (ie healthy cats) can also fail to respond in terms of detectable/significant changes in measured T4.  
  • In normal healthy cats total thyroxine (tT4) increases >50% following administration of TRH.   

Sampling

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Tests

Methodologies

  • Requires validated assays. 
  • Varied Fluorescent and Enzymatic ImmunoAssays (EIAs) or RadioImmunological Assays (IRMA / RIAs) Enzyme linked immunosorbent assay (ELISA)
  • Laboratory testing should ideally use external quality assurance schemes to validate performance. (eg VEEEQAS / ESVE). 

Availability

  • In house and most commercial laboratories (EIAs), few reference laboratories (RIAs). 

Validity

  • Varies with study, test methodology and reference method used, historically often relative to dynamic testing and/or thyroid gland histopathology. 

Sensitivity

  • 98%.  

Specificity 

  • 93%.  

Technique (intrinsic) limitations

  • Side-effects (vomiting, salivation, tachypnea, defecation and occasionally urination) are commonly seen after administration of TRH. 
  • These drug side-effects typically resolve during the 4 hours of the test period, so ideally admit animals for this procedure. 
  • Spectrophotometric in-house analyzers are often more prone to hemolytic and lipemic interference-artifact.  
  • Minimal otherwise if test set up and QC material performed well. 

Technician (extrinsic) limitations

  • Extravasted TRH rather than intravenous, so no resulting stimulation. Test will need repeating if physically noted.

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Peterson M E, Broussard J D & Gamble D A (1994) Use of the thyrotropin-releasing hormone stimulation test to diagnose mild hyperthyroidism in cats. JVIM 4, 279-286 (good controlled study evaluating test.)
  • Sparkes A K, Jones B R, Gruffyd Jones T J & Walker M J (1991) Thyroid function in the cat - Assessment by the TRH response test and the thyrotropin stimulation test. JSAP 32, 59-63.

Other sources of information 

  • Peterson M E (2000) Hyperthyroidism. In: Ettinger S J, Feldman E C (eds): Textbook of Veterinary Internal Medicine; diseases of the Dog and Cat. 5th edn. Philadelphia, W B Saunders Co., pp 1400-1419. 
  • Feldman and Nelson's (2015) Canine and Feline Endocrinology and Reproduction. 4th edn, Elsevier Science, USA. 


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