Circannual variability in adrenocorticotropic hormone responses to administration of thyrotropin-releasing hormone in clinically normal horses in Australia

• Published in: The veterinary journal (1997) Vol. 238; pp. 58 - 62
• Main Authors: Byrne, D.P., Secombe, C.J., Tan, R.H.H., Perera, D.I., Watts, S.P., Wearn, J.G.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-08-2018
• Subjects: Adrenocorticotropic hormone; Equine; Pituitary pars intermedia dysfunction; Reference values; Thyrotropin-releasing hormone; Equine; Adrenocorticotropic hormone; Thyrotropin-releasing hormone; Pituitary pars intermedia dysfunction; Reference values
• ISSN: 1090-0233; 1532-2971
• DOI: 10.1016/j.tvjl.2018.07.008

•Thyrotropin releasing hormone-stimulation tests were performed twice at two locations in Australia.•The results between the winter and summer solstices were similar to those in the northern hemisphere.•The results between the summer and winter solstices were highly variable in healthy horses.
Thyrotropin releasing hormone (TRH)-stimulation testing for pituitary pars intermedia dysfunction (PPID) in horses is only recommended at certain times of the year. Current diagnostic cut-off values reflect testing in the northern hemisphere during this time. The aims of this study were to evaluate TRH stimulation testing during two different phases of the circannual pituitary cycle and to determine whether diagnostic cut-off values developed in the northern hemisphere are appropriate in Australia. Thirteen clinically normal horses at Perth, Western Australia, and 23 horses at Townsville, Queensland, Australia, had TRH stimulation tests performed at two different time points during the circannual pituitary cycle. At both locations, post-TRH adrenocorticotropic hormone (ACTH) concentrations were significantly different between testing time points (Perth: P=0.001; Townsville: P<0.0001). In Perth, the mean ACTH concentrations 10min post-TRH in September and March were 51.4pg/mL (95% confidence interval, CI, 46.4–56.4pg/mL) and 248.5pg/mL (95% CI 170.2–326.9pg/mL), respectively. The median percentage change in ACTH concentrations in March was 361.9%. In Townsville, the mean ACTH concentrations 30min post-TRH in September and April were 35.3pg/mL (95% CI 29.6–40.9pg/mL) and 112.3pg/mL (95% CI 93.4–131.2pg/mL), respectively. The median percentage change in ACTH concentrations in April was 144.7%. The ACTH cut-off value after TRH stimulation in normal horses in September in Perth and Townsville was similar to the values established in the northern hemisphere. However, TRH stimulation testing in March/April was highly variable at both locations.