Adrenocorticotropin concentration following administration of thyrotropin-releasing hormone in healthy horses and those with pituitary pars intermedia dysfunction and pituitary gland hyperplasia.

The research investigated the effect of thyrotropin-releasing hormone (TRH) on adrenocorticotropin (ACTH) concentrations in horses, specifically comparing healthy horses and those with pituitary conditions. The researchers used this comparison to evaluate the effectiveness of the TRH test as a diagnostic tool for pituitary disorders.

Methodology

• The researchers used a prospective case series involving a number of horses in different health conditions: 15 horses exhibiting signs of pituitary pars intermedia dysfunction (PPID), 4 with equivocal symptoms of PPID, and 29 healthy horses showing no signs of PPID.
• They measured ACTH concentrations before and after the administration of TRH a total of 61 times in 48 horses.
• They then compared the results of the TRH test, which measures ACTH response, to the dexamethasone suppression test (DST), which measures cortisol response, in 29 of the horses.
• Additionally, 33 horses (including healthy ones and those with PPID symptoms) were euthanized and their pituitary glands were examined post-mortem.

Results

• ACTH concentrations increased in all horses after the administration of TRH. However, the degree and duration of the increase were significantly higher in horses with PPID.
• They found that the ACTH baseline concentrations and the response to TRH administration did not correlate with the results of the DST.
• The researchers also observed that endogenous ACTH concentrations varied by season.
• While results of DST were abnormal only in clinically abnormal horses or those with pars intermedia hyperplasia, the test produced results within the reference range in 17 of 26 corresponding examinations.

Conclusions

• The researchers concluded that the ACTH response to TRH administration is a useful diagnostic tool for identifying pituitary gland hyperplasia, particularly in horses with baseline ACTH concentrations within the reference range.
• Conversely, the DST, which measures cortisol response, only showed good specificity but poor sensitivity and inconsistent results. Therefore, it did not often align with the results of the TRH test.