Results of a combined dexamethasone suppression/thyrotropin-releasing hormone stimulation test in healthy horses and horses suspected to have a pars intermedia pituitary adenoma.

The research evaluates the combined use of dexamethasone and thyrotropin-releasing hormone (TRH) in diagnosing a certain type of pituitary gland tumor called pars intermedia pituitary adenoma (PIPA) in horses.

Introduction

Pars intermedia pituitary adenoma (PIPA) is a type of pituitary gland tumor. In this study, the researchers used a combined test of dexamethasone suppression and thyrotropin-releasing hormone (TRH) stimulation to identify the presence of PIPA in horses.

Method

• The study was a case-control type, involving seven healthy adult horses and five horses suspected to have PIPA.
• A baseline blood sample was collected from each horse.
• Subsequently, each horse was given an intravenous (IV) injection of dexamethasone at a dose of 40 micrograms/kg of body weight.
• A second blood sample was collected three hours after the dexamethasone injection. After this, TRH was administered intravenously at a dose of 1.1 mg.
• After the TRH injection, blood samples were collected at 15, 30, 45, 60, and 90 minute intervals, and then 21 hours later, which marked 24 hours after the initial dexamethasone injection.
• The cortisol concentration in each blood sample was determined as cortisol levels can indicate the presence of PIPA.

Results

• At baseline, the cortisol concentration was found to be significantly lower in horses suspected of having PIPA than in healthy horses.
• The dexamethasone suppressed cortisol concentration in both groups of horses.
• However, following the administration of TRH, cortisol levels returned to baseline values in horses suspected of having PIPA, but not in healthy horses. This suggests a distinct reaction in horses with PIPA compared to healthy horses.
• In healthy horses, the cortisol concentration was still lower than the baseline value even 24 hours after the dexamethasone injection.

Conclusion

Based on the results of this study, the combined test of dexamethasone suppression and TRH stimulation could be a reliable diagnostic procedure for horses suspected to have PIPA. The research recommends that a blood sample be collected for cortisol level determination 30 minutes after TRH administration when this test is applied in a clinical setting.