Influence of feeding and other factors on adrenocorticotropin concentration and thyrotropin-releasing hormone stimulation test in horses and ponies

Abstract

Background: The basal (bACTH) and post-thyrotropin-releasing hormone stimulation concentration of adrenocorticotropin (pACTH) are recommended for diagnosis of pituitary pars intermedia dysfunction (PPID). Many factors influence bACTH (e.g., disease, age, month) and some affect the results only in autumn (e.g., breed, colour, sex). There are discrepancies about the impact of feeding on b/pACTH. Objectives: To determine whether feeding, month, age, breed, colour, sex and body condition score affect b/pACTH. Study design: Prospective crossover. Methods: Sixty-one animals were divided into groups: healthy, PPID, treated-PPID. The b/pACTH was measured three times (1 mg protirelin; blood collection after 10 min; mid-November to mid-July) after different feedings: fasting, hay, hay + grain. Friedman's test was applied to evaluate the influence of feeding on b/pACTH and linear mixed model to evaluate impact of further factors. Results: The b/pACTH was not significantly affected by feeding (p = 0.7/0.5). The bACTH was lowest in healthy (29.3 pg/mL, CI 9–49.5 pg/mL) and highest in PPID-group (58.9 pg/mL, CI 39.7–78.1 pg/mL). The pACTH was significantly lower in healthy (396.7 pg/mL, CI 283.2–510.1 pg/mL) compared to PPID (588.4 pg/mL, CI 480.7–696.2 pg/mL) and treated-PPID group (683.1 pg/mL, CI 585.9–780.4 pg/mL), highest in July (881.2 pg/mL, CI 626.3–1136.3 pg/mL) and higher in grey (723.5 pg/mL, CI 577.5–869.4 pg/mL) than other colours (338.7 pg/mL, CI 324.8–452.5 pg/mL). The size of effect for those variables was >0.5. Main limitations: Small number of animals, subsequent bACTH measurements were significantly lower in each horse. Conclusions: There was no evidence that feeding influences the b/pACTH. There was evidence that pergolide affects the bACTH but it had little effect on pACTH. Further investigation of the impact of month and coat colour on b/pACTH is warranted to better interpret the results.