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α-Melanocyte—stimulating hormone and adrenocorticotropin concentrations in response to thyrotropin-releasing hormone and comparison with adrenocorticotropin concentration after domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction

Jill BeechDepartment of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348.

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Dianne McFarlaneCenter for Veterinary Health Sciences, Oklahoma State University, Stillwater, OK 74078.

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Sue LindborgDepartment of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348.

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Janice E. SojkaDepartment of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

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Raymond C. BostonDepartment of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348.

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Abstract

Objective—To compare endogenous ACTH and α-melanocyte-stimulating hormone (α-MSH) concentrations after administration of thyrotropin-releasing hormone (TRH) and to compare ACTH concentrations after TRH administration with those following domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction (PPID).

Design—Prospective case series.

Animals—69 clinically normal horses and 47 horses with or suspected to have PPID.

Procedures—ACTH concentrations were measured during 108 TRH stimulation tests in 88 horses, and α-MSH concentrations were measured during 56 TRH stimulation tests in 50 horses. In 28 of these horses, ACTH concentrations after domperidone administration were measured and test results were compared. The pituitary gland was histologically examined in all horses that were euthanatized.

Results—ACTH and α-MSH concentrations increased in all horses afterTRH administration, with a greater and more prolonged increase in horses with PPID. Percentage increase was significantly greater for α-MSH concentration than for ACTH concentration. The change in ACTH concentration after domperidone administration was less consistent in differentiating clinically normal horses from those with PPID than was the response to TRH.

Conclusions and Clinical Relevance—Results suggested that ACTH concentration in response to TRH administration was useful for the diagnosis of PPID in horses and appeared more accurate than response to domperidone administration. Use of an α-MSH concentration ≥ 30 or 50 pmol/L did not appear superior to use of an ACTH concentration ≥ 36 pg/mL for the diagnosis of PPID, either before or 30 minutes after TRH administration.

Abstract

Objective—To compare endogenous ACTH and α-melanocyte-stimulating hormone (α-MSH) concentrations after administration of thyrotropin-releasing hormone (TRH) and to compare ACTH concentrations after TRH administration with those following domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction (PPID).

Design—Prospective case series.

Animals—69 clinically normal horses and 47 horses with or suspected to have PPID.

Procedures—ACTH concentrations were measured during 108 TRH stimulation tests in 88 horses, and α-MSH concentrations were measured during 56 TRH stimulation tests in 50 horses. In 28 of these horses, ACTH concentrations after domperidone administration were measured and test results were compared. The pituitary gland was histologically examined in all horses that were euthanatized.

Results—ACTH and α-MSH concentrations increased in all horses afterTRH administration, with a greater and more prolonged increase in horses with PPID. Percentage increase was significantly greater for α-MSH concentration than for ACTH concentration. The change in ACTH concentration after domperidone administration was less consistent in differentiating clinically normal horses from those with PPID than was the response to TRH.

Conclusions and Clinical Relevance—Results suggested that ACTH concentration in response to TRH administration was useful for the diagnosis of PPID in horses and appeared more accurate than response to domperidone administration. Use of an α-MSH concentration ≥ 30 or 50 pmol/L did not appear superior to use of an ACTH concentration ≥ 36 pg/mL for the diagnosis of PPID, either before or 30 minutes after TRH administration.

Contributor Notes

Supported by M. Gardiner and the American College of Veterinary Internal Medicine Foundation. Domperidone was donated by Equi-Tox Incorporated.

The authors thank Drs. Perry Habecker and Peg Miller for pituitary gland evaluations and Dr. Barbara Schanbacher and Stephen Lamb for technical assistance.

Address correspondence to Dr. Beech (jbeech@vet.upenn.edu).