Repeatability of a thyrotropin-releasing hormone stimulation test for diagnosis of pituitary pars intermedia dysfunction in mature horses.
Authors: Kam Yan Ning, McKenzie Kelly, Coyle Mitchell, Bertin François-René
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Repeatability of TRH Stimulation Testing for PPID Diagnosis Pituitary pars intermedia dysfunction (PPID) diagnosis relies on the thyrotropin-releasing hormone (TRH) stimulation test, yet the seasonal reliability of this important diagnostic tool has been unclear until now. Researchers performed repeat TRH stimulation tests (1 mg IV with blood sampling before and 30 minutes post-injection) on 20 mature horses across two seasons, with 3 horses tested in both autumn and winter, measuring ACTH concentrations via chemiluminescent assay and analysing test-retest reliability using intraclass correlation coefficients and within-horse coefficients of variation. Winter testing demonstrated excellent repeatability with no significant variation between tests performed one week apart and only 2 misclassifications, whilst autumn testing proved considerably less reliable, with significantly lower post-TRH ACTH concentrations on the second test occasion (P = .02), larger within-horse variability (P = .04), and 4 misclassifications across the cohort. These findings suggest that clinicians should exercise caution when interpreting TRH stimulation tests performed during autumn, as the increased physiological variability during this season—likely reflecting changing photoperiod and metabolic state—substantially increases the risk of diagnostic error. Practitioners may benefit from performing repeat testing during more stable seasons, or alternatively, considering complementary diagnostic approaches such as overnight dexamethasone suppression testing if autumn diagnosis of PPID is clinically necessary.
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Practical Takeaways
- •TRH stimulation testing for PPID diagnosis is more reliable when performed during winter months; consider repeat testing or alternative diagnostic approaches if initial testing occurs in autumn
- •Results interpretation must account for seasonal variation, with greater variability in ACTH responses occurring in autumn potentially leading to misdiagnosis
- •Clinicians should be aware that a single positive TRH test in autumn may warrant confirmation testing, particularly when clinical signs are equivocal
Key Findings
- •TRH stimulation test showed excellent test-retest reliability in winter with no significant week effect detected
- •In autumn, after-TRH ACTH concentrations were significantly lower on week 2 (P=0.02) with only good test-retest reliability
- •Within-horse coefficients of variation were significantly larger during autumn (P=0.04) and after TRH stimulation (P=0.04)
- •Four misclassifications occurred in autumn compared to two in winter, indicating seasonal variation affects diagnostic accuracy