Evaluation of Seasonal Changes in Plasma Adrenocorticotropin Hormone (ACTH) Levels in Response to TRH Administration in Horses with and Without Clinical Signs of PPID in the Netherlands

Background
The trH stimulation test is a relatively new and potentially more sensitive diagnostic test for ppiD in horses than previous tests, but clinical evaluation of this test is limited. reference values were previously established in a small group of horses for winter, spring and summer in the uSa.

Hypotheses/goal
The aim of this study was to evaluate seasonal changes in plasma adrenocorticotropin hormone (aCtH) levels in response to thyrotropin releasing Hormone (trH) adminis- tration in horses with clinical signs of ppiD and aged horses without signs of ppiD in the netherlands.

Materials and methods
The trH stimulation test was carried out in 24 horses and 11 ponies in autumn (october) and winter (January) in the netherlands. Synthetic trH (1 mg dissolved in
5 ml naCl 0.9%) was administered iV and plasma aCtH levels were measured before and 10 minutes after trH administration. the animals were allocated to the ppiD (n=14) or control group (n=21) based on presence or absence of typical clinical symptoms (i.e. hypertrichosis).

Results
Plasma concentrations of aCtH increased significantly after trH administration in both groups. aCtH levels before and after trH stimulation were significantly higher for horses with clinical symptoms of ppiD compared to control horses. aCtH levels before and after administration with trH were significantly higher in october than in January for both groups. in the control group, elevated aCtH levels post trH administration outside the reference range were found in 6 of 21 horses.

Conclusions
All ppiD horses responded to administration of trH with aCtH levels outside the reported reference range in winter (>100 pg/ml), which makes the test suitable for con- firming clinical ppiD in this season. this reference value was not suitable for confirming ppiD in october. the 6 control horses with a positive trH stimulation test in January could be early ppiD cases. further research is needed to confirm this.