Adrenocorticotropin concentration following administration of thyrotropin-releasing hormone in healthy horses and those with pituitary pars intermedia dysfunction and pituitary gland hyperplasia.
OBJECTIVE
To compare the effect of thyrotropin-releasing hormone (TRH) administration on endogenous ACTH concentrations in healthy horses and those with pituitary pars inter-media hyperplasia and compare the test with the dexamethasone suppression test (DST).


METHODS
Prospective case series.


METHODS
15 horses with clinical signs of pituitary pars intermedia dysfunction (PPID), 4 horses with equivocal signs of PPID, and 29 horses without signs of PPID.


METHODS
ACTH concentrations prior to and after administration of TRH were measured 61 times in 48 horses. Results of the DST (cortisol response) were compared with those of the TRH test in 29 horses. Thirty-three horses (24 with no clinical signs of PPID, 5 with clinical signs of PPID, and 4 with equivocal clinical signs of PPID) were euthanized and necropsied and their pituitary glands evaluated.


RESULTS
ACTH concentrations increased in all horses, but magnitude and duration of increase were significantly higher in horses with PPID. Endogenous ACTH concentrations were influenced by season. The ACTH baseline concentrations and response to TRH were not correlated with results of the DST. Results of DST were abnormal only in clinically abnormal horses or those with pars intermedia hyperplasia, but were within reference range in 17 of 26 tests in these horses.


CONCLUSIONS
The ACTH response to TRH is a useful test for diagnosis of pituitary gland hyperplasia, particularly in horses in which baseline ACTH concentrations are within reference range. The DST was specific but not sensitive and was inconsistent for individuals, and results often did not agree with the TRH test response.
Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348, USA.
Jill
J
Beech
Raymond
R
Boston
Sue
S
Lindborg
Gail E
GE
Russell
D000324:Adrenocorticotropic Hormone / Q000097:blood*; D000818:Animals; D003907:Dexamethasone / Q000494:pharmacology; D003937:Diagnosis, Differential; D005260:Female; D006728:Hormones / Q000494:pharmacology; D006734:Horse Diseases / Q000097:blood / Q000175:diagnosis*; D006736:Horses; D006854:Hydrocortisone / Q000097:blood; D006965:Hyperplasia / Q000097:blood / Q000175:diagnosis / Q000662:veterinary; D008297:Male; D010900:Pituitary Diseases / Q000097:blood / Q000175:diagnosis / Q000662:veterinary*; D052716:Pituitary Gland, Intermediate / Q000503:physiopathology*; D010911:Pituitary Neoplasms / Q000097:blood / Q000175:diagnosis / Q000662:veterinary*; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D006728:Hormones; D013973:Thyrotropin-Releasing Hormone; D003907:Dexamethasone; D000324:Adrenocorticotropic Hormone; D006854:Hydrocortisone