Circannual variability in adrenocorticotropic hormone responses to administration of thyrotropin-releasing hormone in clinically normal horses in Australia.
Authors: Byrne D P, Secombe C J, Tan R H H, Perera D I, Watts S P, Wearn J G
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Circannual Variability in TRH Stimulation Testing in Australian Horses Thyrotropin-releasing hormone (TRH) stimulation testing serves as a diagnostic tool for pituitary pars intermedia dysfunction (PPID), yet current diagnostic cut-off values were established in the northern hemisphere and are only recommended during specific seasons when pituitary hormone secretion is relatively stable. Byrne and colleagues investigated whether these standardised cut-offs remain valid in Australian horses, which experience an inverse seasonal cycle, by conducting TRH stimulation tests in clinically normal horses at two Australian locations (Perth and Townsville) during different phases of the circannual pituitary rhythm. Post-stimulation ACTH concentrations differed dramatically between testing periods: in Perth, September responses (51.4 pg/mL) were five-fold lower than March responses (248.5 pg/mL), whilst Townsville showed similar disparity between September (35.3 pg/mL) and April (112.3 pg/mL). The practical implication is clear—diagnostic interpretation of TRH stimulation tests in Australia requires region-specific and season-specific cut-off values, as applying northern hemisphere standards to southern hemisphere horses tested during their late summer/autumn months risks misdiagnosis of PPID; testing should ideally be performed during September when ACTH responses align with established reference ranges, or practitioners must adopt modified cut-offs for other seasons.
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Practical Takeaways
- •TRH stimulation testing for PPID diagnosis should preferably be performed in September in Australia to match northern hemisphere reference ranges; testing in March/April may produce false-positive results due to natural seasonal elevation of ACTH
- •Current diagnostic cut-off values developed in the northern hemisphere are not applicable year-round in Australia—practitioners must consider the testing season when interpreting results
- •If PPID testing is necessary outside the recommended September window, use location-specific and season-specific reference intervals rather than standard northern hemisphere cut-offs to avoid misdiagnosis
Key Findings
- •Post-TRH ACTH concentrations differ significantly between seasons in clinically normal horses, with March/April values substantially higher than September values (Perth: 51.4 vs 248.5 pg/mL; Townsville: 35.3 vs 112.3 pg/mL)
- •ACTH responses show marked circannual variability with median percentage changes of 361.9% in Perth (March) and 144.7% in Townsville (April)
- •Northern hemisphere diagnostic cut-off values are appropriate for testing in September but not for March/April testing in Australia
- •TRH stimulation testing in March/April produces highly variable results at both Australian locations, limiting diagnostic reliability during these months