Evaluation of the combined dexamethasone suppression/ thyrotropin-releasing hormone stimulation test for detection of pars intermedia pituitary adenomas in horses

J Vet Intern Med. 2006 Jul-Aug;20(4):987-93. doi: 10.1892/0891-6640(2006)20[987:eotcdt]2.0.co;2.


Background: A combined dexamethasone (DEX) suppression/thyrotropin-releasing hormone (TRH) test (DEX/TRH test) has been developed to evaluate horses for presence of a pars intermedia pituitary adenoma (PIPA), but to the authors' knowledge, the accuracy of this test has not been previously determined.

Hypothesis: The sensitivity and specificity of the DEX/TRH test can be determined by comparing test results with histopathologic examination findings.

Animals: Age of 42 horses of various breeds ranged from 2 to 33 years.

Methods: Plasma cortisol concentration was measured before and 24 hours after IV administration of 40 microg of DEX/kg of body weight, and before and 30 minutes after IV administration of 1 mg of TRH that had been given 3 hours after the injection of DEX. Results of the DEX/TRH test were considered positive if either the plasma cortisol concentration exceeded 10 ng/mL 24 hours after DEX administration, or if the change in plasma cortisol concentration 30 minutes after injection of TRH was > or = 66% above the 3-hour baseline. Diagnosis of PIPA was determined by histologic examination of the pituitary gland.

Results: PIPA was detected in 17 of 42 (40%) horses. The DEX/TRH test had sensitivity, specificity, positive predictive value, and negative (NPV) predictive value of 88, 76, 71, and 90%, respectively.

Conclusions and clinical importance: The combined DEX/TRH test was more sensitive than either of its component tests and had a high NPV, but was not as specific as the TRH component alone (92%). The DEX/TRH test should be used to screen older horses for PIPA.

Publication types

  • Evaluation Study

MeSH terms

  • Animals
  • Dexamethasone*
  • Female
  • Horse Diseases / diagnosis*
  • Horses
  • Male
  • Pituitary Gland / pathology
  • Pituitary Neoplasms / diagnosis
  • Pituitary Neoplasms / veterinary*
  • Sensitivity and Specificity
  • Thyroid Function Tests / veterinary*
  • Thyrotropin-Releasing Hormone*


  • Thyrotropin-Releasing Hormone
  • Dexamethasone