Evaluation of seasonal influences on adrenocorticotropic hormone response to the thyrotropin-releasing hormone stimulation test and its accuracy for diagnosis of pituitary pars intermedia dysfunction.
Authors: Adams A A, Siard-Altman M H, Reedy S E, Barker D, Elzinga S, Sanz M G, Urschel K, Ireland J L
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Seasonal ACTH Response to TRH Stimulation in PPID Diagnosis Pituitary pars intermedia dysfunction affects more than one in five older horses, yet early detection remains diagnostically challenging; the thyrotropin-releasing hormone (TRH) stimulation test has shown promise for identifying mild or subclinical cases, but its seasonal fluctuations have previously limited year-round clinical application. Adams and colleagues conducted a 12-month prospective study in 63 horses (17 controls, 21 with subclinical PPID, and 25 with clinical PPID), performing monthly TRH stimulation tests and deriving diagnostic cut-off values using receiver operating characteristic curve analysis. TRH-stimulated adrenocorticotropic hormone (ACTH) concentrations demonstrated pronounced circannual variation, with lowest values in February–May and highest in August–October, though the test demonstrated superior diagnostic accuracy compared to basal ACTH measurement alone. Critically, whilst TRH stimulation proved considerably more sensitive for detecting subclinical PPID than basal testing, it showed limited additional diagnostic value in clinically obvious cases, and substantial discrepancies existed between cut-off values optimising sensitivity versus specificity. Practitioners should interpret TRH stimulation results within the seasonal context and clinical presentation, recognising that this test is most valuable for identifying early or subtle disease rather than confirming overt PPID, and that diagnostic thresholds will require monthly adjustment for optimal diagnostic performance.
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Practical Takeaways
- •TRH stimulation tests should be interpreted with monthly cut-off values rather than uniform year-round thresholds, with particular caution needed August-October when ACTH responses peak seasonally
- •For suspected early or subclinical PPID, TRH stimulation testing is more sensitive than basal ACTH alone, but always correlate test results with clinical signs rather than relying on results in isolation
- •Consider testing timing strategically: February-May offers the most stable baseline ACTH responses for diagnosis; avoid assumptions about positive results in late summer without considering seasonal variation
Key Findings
- •TRH-stimulated ACTH concentrations were lowest in February-May and highest in August-October, demonstrating marked seasonal variability
- •TRH stimulation test showed improved diagnostic accuracy compared to basal ACTH alone, though sensitivity was not significantly greater year-round
- •Specificity of both basal and 30-minute post-TRH ACTH was generally higher than sensitivity across the study period
- •TRH stimulation yielded considerably more positive results in subclinical PPID cases than basal ACTH testing, but few additional positive results in clinical cases