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


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


METHODS
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
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.
Frank
Nicholas
N
Department of Large Animal Clinical Sciences, University of Tennessee, Knoxville 37996, USA. nfrank@utk.edu
Andrews
Frank M
FM
Sommardahl
Carla S
CS
Eiler
Hugo
H
Rohrbach
Barton W
BW
Donnell
Robert L
RL
D000818:Animals; D003907:Dexamethasone; D005260:Female; D006734:Horse Diseases / Q000175:diagnosis*; D006736:Horses; D008297:Male; D010902:Pituitary Gland / Q000473:pathology; D010911:Pituitary Neoplasms / Q000175:diagnosis / Q000662:veterinary*; D012680:Sensitivity and Specificity; D013960:Thyroid Function Tests / Q000662:veterinary*; D013973:Thyrotropin-Releasing Hormone
D013973:Thyrotropin-Releasing Hormone; D003907:Dexamethasone