Alpha-melanocyte stimulating hormone release in response to thyrotropin releasing hormone in healthy horses, horses with pituitary pars intermedia dysfunction and equine pars intermedia explants
Authors: McFarlane Dianne, Beech Jill, Cribb Alastair
Journal: Domestic Animal Endocrinology
Summary
# Editorial Summary: TRH Stimulation Testing for PPID Diagnosis Pituitary pars intermedia dysfunction (PPID) diagnosis in horses currently relies on various hormonal markers, but the most reliable test protocol remains unclear. McFarlane and colleagues compared how healthy horses and those with PPID respond to thyrotropin-releasing hormone (TRH) administration by measuring both ACTH and alpha-melanocyte-stimulating hormone (α-MSH) concentrations in 69 normal and 47 affected horses, with additional comparison to the dopamine antagonist domperidone in 28 animals. Both hormones increased after TRH injection in all horses, but those with PPID showed significantly greater and more prolonged elevation of both markers, with α-MSH demonstrating a proportionally larger response than ACTH despite both being released from the same pituitary cells. The TRH stimulation test proved more reliable for differentiating healthy from affected horses than domperidone administration, making it the superior diagnostic choice. For practitioners, these findings suggest that the established ACTH cutoff of ≥36 pg/mL following TRH administration remains a solid diagnostic benchmark, and measuring α-MSH adds little additional discriminatory value over ACTH measurement alone, potentially simplifying diagnostic protocols and reducing laboratory costs without compromising diagnostic accuracy.
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Practical Takeaways
- •TRH stimulation testing is a reliable diagnostic tool for PPID, showing clearer differentiation between healthy and affected horses than domperidone testing
- •When interpreting PPID diagnostic tests, ACTH response to TRH is more practical than measuring α-MSH, as ACTH thresholds are better established (≥36 pg/mL)
- •If your horse shows suspected signs of PPID (abnormal coat, laminitis, excessive sweating), request TRH stimulation testing rather than domperidone testing for more accurate diagnosis
Key Findings
- •ACTH and α-MSH concentrations increased in all horses after TRH administration, with significantly greater and more prolonged increases in horses with PPID
- •Percentage increase in α-MSH was significantly greater than ACTH concentration increase after TRH stimulation
- •TRH stimulation test was more consistent at differentiating healthy horses from PPID-affected horses compared to domperidone administration
- •ACTH concentration ≥36 pg/mL after TRH administration was more diagnostically useful than α-MSH concentration ≥30 or 50 pmol/L for PPID diagnosis