OBJECTIVE To assess effects of withholding feed on thyrotropin-releasing hormone (TRH) stimulation test results used in diagnosis of pituitary pars intermedia dysfunction in horses and determine effects of combined testing on results of the TRH stimulation test and the oral sugar test (OST) used in diagnosis of equine metabolic syndrome. ANIMALS 30 adult horses. PROCEDURES All horses underwent TRH stimulation tests under fed and nonfed conditions, an OST alone, and an OST combined with TRH stimulation testing. For TRH stimulation tests, plasma ACTH concentrations were measured before (baseline) and 10 minutes after (poststimulation) IV TRH administration. For the OST, plasma glucose and insulin concentrations were measured before (baseline) and 60 and 90 minutes after oral corn syrup administration. For combined testing, the TRH stimulation test was initiated immediately after 60-minute posttreatment sample collection for the OST. Results were compared among methods by Wilcoxon matched-pairs, signed rank tests, paired t tests, and Bland-Altman analysis. RESULTS Feeding conditions did not affect median ACTH concentrations when TRH stimulation tests were performed alone. Median baseline ACTH concentration did not differ between TRH stimulation tests performed alone (under fed or nonfed conditions) and those combined with OSTs. Median poststimulation ACTH concentration was significantly lower for combined tests than for solitary TRH stimulation tests. Mean 60-minute plasma glucose concentration was significantly lower for solitary OSTs than for combined tests, but this difference could not be attributed to TRH administration. CONCLUSIONS AND CLINICAL RELEVANCE Combined testing in the manner described impacted ACTH concentrations during TRH stimulation tests and is not recommended at this time.

OBJECTIVE To determine effects of anesthesia on plasma concentrations and pulsatility of ACTH in samples obtained from the cavernous sinus and jugular vein of horses. ANIMALS 6 clinically normal adult horses. PROCEDURES Catheters were placed in a jugular vein and into the cavernous sinus via a superficial facial vein. The following morning (day 1), cavernous sinus blood samples were collected every 5 minutes for 1 hour (collection of first sample = time 0) and jugular venous blood samples were collected at 0, 30, and 60 minutes. On day 2, horses were sedated with xylazine hydrochloride and anesthesia was induced with propofol mixed with ketamine hydrochloride. Horses were positioned in dorsal recumbency. Anesthesia was maintained with isoflurane in oxygen and a continuous rate infusion of butorphanol tartrate. One hour after anesthesia was induced, the blood sample protocol was repeated. Plasma ACTH concentrations were quantified by use of a commercially available sandwich assay. Generalized estimating equations that controlled for horse and an expressly automated deconvolution algorithm were used to determine effects of anesthesia on plasma ACTH concentrations and pulsatility, respectively. RESULTS Anesthesia significantly reduced the plasma ACTH concentration in blood samples collected from the cavernous sinus. CONCLUSIONS AND CLINICAL RELEVANCE Mean plasma ACTH concentrations in samples collected from the cavernous sinus of anesthetized horses were reduced. Determining the success of partial ablation of the pituitary gland in situ for treatment of pituitary pars intermedia dysfunction may require that effects of anesthesia be included in interpretation of plasma ACTH concentrations in cavernous sinus blood.