Laminitis in Pituitary Pars Intermedia Dysfunction
Authors: Johnson Philip J.
Journal: Equine Laminitis
Summary
# Editorial Summary: Laminitis in Pituitary Pars Intermedia Dysfunction PPID remains a significant differential diagnosis in laminitis cases, yet diagnostic protocols continue to evolve. Johnson's prospective investigation examined whether the post-/pre-TRH ACTH ratio—a calculated metric derived from TRH stimulation testing—could enhance diagnostic accuracy for PPID compared to using absolute post-stimulation ACTH values alone. Over a 12-month period, 21 affected horses and 63 controls underwent TRH challenge with blood sampling before and 30 minutes after intravenous hormone injection, with statistical analysis revealing that whilst both PPID status and seasonal variation significantly influenced the ratio, diagnostic accuracy fluctuated considerably month-to-month (ranging from fair to good), and positive likelihood ratios remained persistently low. The critical finding for practitioners is that calculating this ratio offers no diagnostic advantage over simply measuring post-TRH ACTH concentration; the additional step adds complexity without improving sensitivity or specificity. For those managing laminitis cases, this reinforces that TRH stimulation testing remains a valid tool for pituitary assessment in PPID evaluation, but practitioners should focus on absolute post-challenge ACTH thresholds rather than ratio calculations, and must account for seasonal variation when interpreting results.
Read the full abstract on PubMed
Practical Takeaways
- •Use TRH stimulation testing to diagnose PPID, but rely on post-TRH ACTH concentration values rather than calculating the pre-/post ratio as it adds no diagnostic benefit
- •Be aware that diagnostic performance of PPID testing varies seasonally, so interpret results in context of the month of testing
- •Confirm PPID diagnosis with traditional post-TRH ACTH thresholds rather than ratio-based cutoffs for more reliable clinical decision-making
Key Findings
- •Post-/pre-TRH plasma ACTH ratio showed significant difference between PPID and control horses in 7 of 12 months studied
- •Diagnostic accuracy of the ratio varied by month with fair to good performance but consistently low positive likelihood ratios
- •The post-/pre-TRH ACTH ratio does not improve diagnostic characteristics compared to post-TRH ACTH concentration alone
- •Both PPID status and month significantly affected the post-/pre-TRH plasma ACTH ratio values