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 g 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 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.
- adenoma adrenal gland diseases blood chemistry Cushing's syndrome dexamethasone diagnosis diagnostic techniques histopathology hydrocortisone neoplasms pituitary diseases thyroid function tests thyrotropin releasing hormone horses adrenal diseases cancers cortisol hyperadrenocorticism pituitary disorders protirelin thyroliberin Equus Equidae Perissodactyla mammals vertebrates Chordata animals ungulates eukaryotes
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