Seasonal Changes in Plasma Adrenocorticotropic Hormone and α‐Melanocyte‐Stimulating Hormone in Response to Thyrotropin‐Releasing Hormone in Normal, Aged Horses

Abstract

Background: Results of diagnostic tests for equine pituitary pars intermedia dysfunction (PPID), including endogenous ACTH concentration and the overnight dexamethasone suppression test (DST), are affected by season. New and potentially more sensitive diagnostic tests for equine PPID, such as thyrotropin-releasing hormone (TRH)-stimulated ACTH response, have been developed, but have had limited evaluation of seasonality.Objective: Our purpose was to evaluate seasonal changes in plasma ACTH and alpha-melanocyte-stimulating hormone (a-MSH) responses to TRH administration.Animals: Nine, healthy, aged horses with normal DST results. Methods: Synthetic TRH (1 mg) was administered IV. Plasma ACTH and a-MSH concentrations were measured at 0,5, 10,15,20, 25,30, 45, 60, and 180 minutes. Testing was performed in February, July, August, September, October, and November. Mean TRH-stimulated ACTH and a-MSH concentrations were compared across months and time by repeated measures analysis of variance. Significance was set at the P o .05 level.Results: Concentrations of ACTH and a-MSH significantly increased after TRH administration. Endogenous and TRHstimulated ACTH and a-MSH concentrations were significantly different across months with higher concentrations in the summer and fall compared with February.Conclusions and Clinical Importance: Plasma ACTH and a-MSH responses to TRH administration experience seasonal variation, with TRH-stimulated ACTH and a-MSH concentrations increasing from summer through fall. These results support previous evidence of a seasonal influence on the equine pituitary-adrenal axis. More research is warranted with a larger number of horses to determine if seasonal reference ranges for TRH stimulation testing need to be defined.

Comparison of Cortisol andACTHResponses after Administration of Thyrotropin Releasing Hormone in Normal Horses and Those with Pituitary Pars Intermedia Dysfunction

“…However, there is no evidence to suggest an effect or indicate that the seasonal effect on ACTH concentration in response to TRH administration explains the different results among tests evaluating cortisol concentration response. The effect on ACTH concentration is modest, and ACTH and cortisol concentrations appear disassociated in horses with PPID. Also, in the current study, exclusion of the 3 horses tested between August and October did not alter results of the test.…”

Pituitarypars intermediadysfunction: Diagnosis and treatment

“…; 2011a,b; Funk et al . ) and, as no seasonal reference intervals are yet published, the test cannot be properly interpreted at that time of year. The recommended procedure for TRH stimulation testing is outlined in Table .…”

Use of the chasteberry preparation Corticosal® for the treatment of pituitary pars intermedia dysfunction in horses

“…Plasma ACTH levels are higher in early fall (August to October) due to a reduction of daylight and decline in winter, reaching their lowest levels in January (Beech et al 2009, Copas et al 2012, Elliott 2010. In a recent study performed by Funk et al (2011), a stronger reaction of ACTH levels after TRH stimulation in July than in February was seen.…”