Equine PPID, also known as equine Cushing's disease, has long been recognized as a syndrome in middle-aged and older horses. The classic clinical signs, including long coat, failure to shed normally, abnormal fat deposits, laminitis, lethargy, and muscle wasting, are well recognized, but there has been increasing interest in diagnosis of the condition in horses prior to onset of severe, sometimes life-threatening, and irreversible signs. Increased basal endogenous ACTH concentration is characteristic of PPID; however, ACTH concentration is not consistently increased and may be influenced by season and stress.1–4 Suppression of endogenous cortisol concentration via administration of dexamethasone was long regarded as the optimal test,5,6 but results can be inconsistent and even variable within the same horse with PPID.3,a Lack of diurnal variation in endogenous cortisol concentration is not a reliable test for PPID because of the many factors that can affect cortisol concentrations and thereby affect interpretation of 2 measurements. The increase in cortisol concentration after TRH administration has been reported7 to be higher in horses with PPID, but this may not consistently differentiate normal from abnormal horses.8,9 The combined dexamethasone suppression-TRH stimulation test was reported10 to have higher sensitivity and accuracy than either of its components, although specificity was lower than that of the TRH component and the same as that of the dexamethasone suppression component.11 As cortisol concentration indirectly represents TRH-induced ACTH release and there are now commercial assays for ACTH concentration, changes in ACTH concentration should be measured when performing the TRH stimulation test.
Because some horses with PPID have basal concentrations of ACTH within reference limits, tests that evoke a response from cells in the pars intermedia appear promising as diagnostic tests for PPID. The ACTH response to TRH administration (1 mg, IV) was reported3 to be useful in diagnosis of PPID, particularly in horses in which endogenous ACTH concentrations were within reference limits. Thyrotropin-releasing hormone has been shown to stimulate the equine pituitary gland and release POMC-derived peptides, including α-MSH release from the pars intermedia and ACTH release from the pars distalis, and TRH receptor RNA is expressed in both the pars intermedia and pars distalis of horses.9 Domperidone is a dopamine receptor antagonist. It has been hypothesized that its action blocking D2 receptors of melanotropes in the pars intermedia would remove dopaminergic inhibition of ACTH release and that horses with PPID would have a greater increase in ACTH secretion than would clinically normal animals.12,13 Two studies12,13 have reported a greater increase in ACTH concentration in horses with PPID than in clinically normal horses 4 and 8 hours after administration of 2.5 and 3.3 mg of domperidone/kg (1.1 and 1.5 mg of domperidone/lb). On the basis of testing additional animals, a modification of the original domperidone test protocol was suggested (5.0 to 5.5 mg/kg [2.3 to 2.5 mg/lb] with blood samples obtained at 0, 2, and 4 hours).b As domperidone administered by mouth is approved for treating agalactia in mares and may be more readily available than TRH, its use has potential advantages compared with the IV administration of TRH. An objective of the study reported here was to compare the ACTH response to TRH administration with the ACTH response to domperidone administration in normal horses and those with PPID to ascertain whether 1 of these tests is diagnostically superior. To our knowledge, a comparison of ACTH concentration changes in response to the domperidone and TRH stimulation tests has not been published.
Additionally, it has been suggested that measuring α-MSH concentration may be superior to measuring ACTH concentration when evaluating pituitary function in horses, although normal seasonal increases must be considered, especially in ponies.19,14,16,c An early studyc that did not account for seasonal variation and used the dexamethasone suppression test to differentiate normal from abnormal horses suggested an α-MSH concentration of > 91 pmol/L was diagnostic for PPID. Later studies have suggested that plasma α-MSH concentrations of > 19 pmol/L or > 25.3 pmol/L in winter, spring, or summer and a value of > 148 pmol/L in autumn14,15 were diagnostic for PPID. An increase in plasma α-MSH concentration after TRH administration has been previously demonstrated in horses.9 However, in the present study, ACTH and α-MSH concentrations were evaluated at additional time periods to ascertain whether measuring α-MSH concentration is superior to measuring ACTH concentration in the diagnosis of PPID. An additional objective was to examine the responses in horses with mild signs of pituitary dysfunction because the development of a test useful for the diagnosis of pituitary dysfunction prior to the onset of severe signs would be clinically relevant for equine medicine. Horses without signs of endocrine disease but with histologic evidence of pituitary changes (hyperplasia or adenomas) were also examined in an attempt to increase our knowledge of the potential functional significance of hypertrophic changes in the pituitary pars intermedia of horses.
Materials and Methods
Animals—Eighty-eight horses (including 4 ponies and 3 pony crosses) were included in the study. All procedures were approved by the Institutional Animal Care and Use Committees of the University of Pennsylvania and Purdue University, and owners of privately owned animals provided informed consent for the tests. All horses, except 4 tested at Purdue University, Lafayette, Ind, were located in Chester County, Pa. The clinically normal horses that underwent necropsy examinations were euthanatized for reasons unrelated to endocrine or systemic disease. In addition to clinical status, horses were classified according to histologic description of the pituitary gland. A gross postmortem examination was performed, and midsagittal sections of the pituitary gland were examined histologically after fixation in neutral-buffered 10% formalin. Horses were classified as having histologic changes in the pituitary pars intermedia (hyperplasia or adenoma) if a sagittal section within 1 mm of the midline that included a portion of the third ventricle had discrete aggregates of cells displaced from the pars intermedia or projecting into the pars nervosa. In some horses, focally abnormal areas or circumscribed adenomas compressing the pars nervosa were grossly visible on histologic sections. Clinically normal horses had no signs of endocrine or systemic disease and also were subclassified on the basis of whether they had had a necropsy performed and whether histologic changes were present or absent in the pituitary gland.
Horses were classified as having PPID if they had at least 2 of the classic clinical signs of PPID, such as abnormal coat and shedding, abnormal fat deposition (over the hindquarters, along the dorsum of the neck, near the tail head, or in the supraorbital fossae), lethargy, loss of epaxial muscle mass, pendulous abdomen, laminitis, or recurrent infections. Thirteen of the PPID horses did not have a necropsy performed, but because all horses with PPID that had a necropsy had a pituitary adenoma, all horses with PPID were combined into 1 group (PPID group). Horses were suspected to have PPID if they had ≥ 1 sign of PPID other than a wavy long coat or improper shedding, had had a necropsy, and were confirmed to have a pituitary adenoma.
TRH stimulation test—For the TRH stimulation test, ACTH concentration was evaluated in 25 horses with PPID, 10 horses suspected to have PPID, 23 clinically normal horses with no histologic pituitary changes, 10 clinically normal, non-necropsied horses, and 20 clinically normal horses with histologic pituitary changes (pituitary hyperplasia or adenomas). The clinically normal, non-necropsied horses consisted of 5 castrated males, 1 sexually intact male, and 4 females and ranged in age from 4 to 11 years (mean ± SD, 8 ± 2 years; median, 8 years). The clinically normal horses with no histologic pituitary changes consisted of 17 castrated males and ± females and ranged in age from 5 to 18 years (mean ± SD, 8 ± 4 years; median, 7 years). The clinically normal horses confirmed to have histologic pituitary changes consisted of 11 castrated males and 9 females and ranged in age from 5 to 29 years (mean ± SD, 16 ± 7 years; median, 16 years). The PPID group consisted of 9 castrated males and 16 females and ranged in age from 8 to 30 years (mean ± SD, 21 ± 7 years; median, 21 years). The horses suspected to have PPID consisted of 7 castrated males and 3 females and ranged in age from 11 to 24 years (mean ± SD, 16 ± 5 years; median, 15 years). Breeds represented included Thoroughbred and Thoroughbred crosses, Quarter Horses, Standardbreds, Arabians, warmbloods and warmblood crosses, Morgan Horses, and ponies. No breed predominated in any group, except among the clinically normal horses with no histologic pituitary changes, for which Thoroughbred and Thoroughbred crosses predominated. Also, 4 of the 7 ponies or pony crosses were in the PPID group. Some horses had > 1 TRH stimulation test. In the PPID group, 1 horse was tested 5 times, 2 were tested 4 times each, and 2 were tested 2 times. In the group of clinically normal horses that had not had a necropsy performed, 1 horse was tested 5 times, 1 was tested 3 times, and 1 was tested twice. One of the clinically normal horses with histologic pituitary changes had 2 tests performed. Concentration of α-MSH was evaluated in a portion of the TRH stimulation tests in 15 horses with PPID, 9 horses suspected to have PPID, 15 clinically normal horses that had no histologic pituitary changes, 2 clinically normal non-necropsied horses, and 9 clinically normal horses with histologic pituitary changes. In the clinically normal non-necropsied horses, α-MSH concentration was measured during 3 tests in one horse and 2 tests in another. In the PPID group, α-MSH concentration was measured during 2 tests in 3 horses. Because the testing occurred over several years, 5 clinically normal horses that had not had a necropsy performed were later confirmed to have histologic pituitary changes. Forty-eight of the horses were included in a previous study.3
For the TRH stimulation test, an 18-gauge IV catheterd was aseptically placed in a jugular vein 30 minutes before TRH administration. Baseline venous blood samples were obtained 5 minutes apart prior to injecting 1 mg of synthetic TRHe and flushing the catheter with 10 mL of heparinized saline (0.9% NaCl) solution. Two baseline samples were obtained from all horses, except for 1 horse from which only 1 sample was obtained for measurement of ACTH concentration and 3 horses for which only 1 sample was obtained for measurement of baseline α-MSH concentration. When 2 samples were obtained, the mean value was used as the 0-minute value. The TRH was reconstituted via sterile technique under a biohazard hood with sterile Dulbecco PBS solutionf to achieve a concentration of 1 mg of TRH/mL. One-milliliter aliquots were filtered with a 0.22-μm syringe filterg and stored in sterile Eppendorf tubes at −70°C until use. Subsequent blood samples were obtained at 4, 14, 30, and 60 minutes after injection of TRH, and the catheter was flushed with heparinized saline solution between each sample. A volume of 6 mL of blood was aspirated and discarded prior to obtaining each blood sample.
Domperidone stimulation test—In 28 of the horses, domperidoneh was administered and ACTH response was evaluated in 8 horses with PPID, 4 horses suspected to have PPID, 8 clinically normal horses that had no histologic pituitary changes, 2 clinically normal horses that had not had a necropsy performed, and 6 clinically normal horses with histologic pituitary changes. One horse with PPID was tested twice. Two clinically normal, non-necropsied horses were castrated males between 9 and 10 years of age. The 8 clinically normal horses that had no histologic pituitary changes were 4 castrated males and 4 females, and age ranged from 2 to 18 years (mean ± SD, 9 ± 6 years; median, 7 years). The group of clinically normal horses with histologic pituitary changes consisted of 3 castrated males and 3 females ranging from 18 to 29 years of age (23 ± 5 years; median, 22 years). The PPID group consisted of 2 castrated males and 6 females, and age ranged from 14 to 34 years (25 ± 7 years; median, 26 years). Two castrated males and 2 females were suspected to have PPID; age ranged between 11 and 17 years (15 ± 3 years; median, 16 years). No breed predominated in any group except in the group of 8 clinically normal horses that had no histologic pituitary changes in which there were 3 warmbloods or warmblood crosses and 4 Thoroughbred or Thoroughbred crosses. For the oral domperidone stimulation test, dosage varied over the time period. Domperidone (2.0 to 2.2 mg/kg) was administered to 2 horses with PPID, 1 clinically normal horse with histologic pituitary changes, 1 clinically normal horse that had no histologic pituitary changes, and 1 horse suspected to have PPID. In 1 clinically normal horse that had not had a necropsy performed, 1 clinically normal horse that had no histologic pituitary changes, and 1 clinically normal horse with histologic pituitary changes, 3.3 mg/kg was given, and 5.0 to 5.5 mg/kg was given to 6 horses with PPID, 1 clinically normal horse that had not had a necropsy performed, 6 clinically normal horses that had no histologic pituitary changes, 4 clinically normal horses with histologic pituitary changes, and 3 horses suspected to have PPID. The drug was administered between 8 am and 10 am, and a blood sample was obtained via jugular venipuncture before and at 2 and 4 hours after administration. The mean ACTH concentrations were compared among groups at 0, 2, and 4 hours. Eleven horses also had blood samples obtained 8 hours after domperidone was administered, but these results were not included in the analyses.
Endocrine assays—All blood samples were collected into evacuated glass tubes containing EDTA as an anticoagulant and centrifuged within 2 hours, and the plasma was transferred to polypropylene containers and frozen at −70°C prior to assay to measure ACTH concentration. For the ACTH assays, frozen samples were placed on ice packs and sent overnight to the Animal Health Diagnostic Center at Cornell University for testing. A sequential immunometric assay that used chemiluminescence for signal generation was used to measure ACTH concentrations.i The assay for ACTH is generally specific for ACTH fragment 1–39 but has approximately 12% to 14% cross-reactivity with fragment 18–39. For the α-MSH assay, frozen samples were sent on dry ice to the laboratory, and concentration was measured by use of a commercially available radioimmunoassayj with a sensitivity of 3 pmol/L and intra- and interassay variation of 5% for both high and low concentrations.2 The laboratory reference range was 9 to 35 pg/mL for ACTH concentration. For α-MSH concentration, 2 values (30 and 50 pmol/L) were evaluated for defining the upper limit of the reference range.
Statistical analysis—Statistical analyses were performed as described.3 Dichotomous outcomes (eg, clinically abnormal horses with pituitary changes vs normal horses with no pituitary changes) were examined with regard to ACTH or α-MSH concentration status at the relevant postchallenge times by use of logistic regression. Sensitivity and specificity of the predictions from these regressions were additionally explored. The Kruskal-Wallis test was used for comparison of percentage changes in ACTH and α-MSH concentrations in the various groups at 4, 14, and 30 minutes after TRH administration. For continuous data (ie, plasma ACTH and α-MSH concentrations), log transformation for normality was applied and confirmed by use of the Shapiro-Wilks test. Repeated-measures regression analysis was used to associate transformed outcome with time and clinical categories. For specific comparisons of continuous outcomes (eg, ACTH concentration for the domperidone stimulation test) with multiples of baseline concentration (eg, ACTH concentration > twice baseline ACTH concentration), both paired comparisons and the nonparametric signed rank test were applied. The more conservative signed rank test results served as a guide to the robustness of the parametric paired comparison test. All statistical analyses were performed with a commercial software package.k Significance was set at a value of P < 0.05 except when the Bonferroni test was applied, and a cutoff value of P < 0.003 was used. The group of clinically normal, non-necropsied horses was not statistically compared as a single group with other groups because the necropsy status was unknown; it was combined with the other clinically normal horses for making comparisons.
Data were not evaluated for seasonal effect because of unequal distribution over the year. However, the data collected between July 21 and October 2 were scrutinized, as concentrations of ACTH and notably α-MSH are typically higher in both normal horses and in those with PPID during summer and autumn versus in winter and spring.1–4,14,l,m When high values of ACTH or α-MSH occurred in normal horses during this time, the date of collection was noted.
Results
TRH stimulation test—Concentrations of ACTH and α-MSH increased after TRH administration (Table 1). Both ACTH and α-MSH concentrations increased rapidly and peaked prior to 14 minutes. Adrenocorticotropin and α-MSH concentrations were higher in PPID horses than in clinically normal horses at baseline and at 4, 30, and 60 minutes after TRH administration. Concentration of α-MSH was also higher in PPID horses than in clinically normal horses at 14 minutes. Concentrations of ACTH and α-MSH were significantly (P < 0.05) higher in horses with PPID combined with horses suspected to have PPID, compared with clinically normal horses, at baseline and at 4, 30, and 60 minutes. When ACTH and α-MSH concentrations were log converted, there were significant (P < 0.05) differences at all times between horses with PPID and clinically normal horses and between horses with PPID combined with horses suspected to have PPID and clinically normal horses. At 4 and 30 minutes after TRH administration, the ACTH and α-MSH concentrations and their log values were significantly (P ≤ 0.003) higher in the horses suspected to have PPID than in the clinically normal horses that had no histologic pituitary changes. At 30 and 60 minutes, log ACTH concentrations were significantly (P ≤ 0.003) higher in the clinically normal horses with histologic pituitary changes than in the clinically normal horses that had no histologic pituitary changes. Comparison of percentage increases in ACTH concentration and α-MSH concentration at 4, 14, and 30 minutes after TRH administration indicated that the percentage increase in α-MSH concentration was significantly (P < 0.05) greater than the increase in ACTH concentration in all groups (Table 2). Comparing the various groups to differentiate abnormal from normal horses, the odds ratio, sensitivity, specificity, and percentage with a correct diagnosis by use of an ACTH concentration > 36 pg/mL and α-MSH concentration ≥ 30 pmol/L and > 50 pmol/L were determined at baseline and 30 minutes after TRH administration (Table 3). At time 0, the odds ratio was significantly (P < 0.05) higher for use of ACTH concentration ≥ 36 pg/mL than use of α-MSH concentration ≥ 30 pmol/L or > 50 pmol/L, except when the horses with PPID or PPID horses combined with horses suspected to have PPID were compared with all clinically normal horses and a concentration of α-MSH > 50 pmol/L was used. When ACTH concentration ≥ 36 pg/mL was used, the odds ratio was significantly (P < 0.05) higher at 30 minutes than at time 0 for differentiating the clinically normal horses from the horses with PPID and from PPID horses combined with horses suspected to have PPID. At 30 minutes, α-MSH concentration exceeded 30 pmol/L in all the horses with PPID, preventing comparison of odds ratios, sensitivity, or specificity with other groups.
Mean, median, and log ACTH (n = 88 horses) and α-MSH concentrations (50) in response toTRH administration (1 mg, IV).
| ACTH (pg/mL) | α-MSH (pmol/L) | |||||
|---|---|---|---|---|---|---|
| Group | 0 min | 4 min | 30 min | 0 min | 4 min | 30 min |
| Clinically normal horses | ||||||
| A (23,15) | ||||||
| Mean ± SD | 23.4 ± 9.3 | 43.3 ± 25.3 | 26.2 ± 8.8 | 11.8 ± 12.5 | 59.9 ± 71.7 | 22.7 ± 26.8 |
| Median | 20.4 | 32.5 | 35.5 | 8.1 | 42.0 | 14.9 |
| Log ± SD | 3.10 ± 0.29 | 3.62 ± 0.45 | 3.22 ± 0.31 | 2.05 ± 1.03 | 3.76 ± 0.74 | 2.75 ± 0.85 |
| B (10, 2) | ||||||
| Mean ± SD | 25.8 ± 12.8 | 117.2 ± 69.6 | 41.5 ± 236 | 26.9 ± 30.3 | 132.± 685.5 | 59.5 ± 0.97 |
| Median | 20.5 | 121 | 30.8 | 14.8 | 129.4 | 36.6 |
| Log ± SD | 3.16 ± 0.43 | 4.54 ± 0.79 | 3.58 ± 0.59 | 2.81 ± 1.06 | 4.63 ± 0.84 | 3.69 ± 0.97 |
| C (20, 9) | ||||||
| Mean ± SD | 41.7 ± 32.9 | 138.8 ± 142.1 | 49.2 ± 26.6 | 20.3 ± 8.6 | 281.1 ± 262.0 | 85 ± 92.1 |
| Median | 24.4 | 89.7 | 42.4 | 20.3 | 227.3 | 61.0 |
| Log ± SD | 3.50 ± 0.68 | 4.62 ± 0.79 | 3.75 ± 0.60 | 2.92 ± 0.46 | 5.25 ± 0.94 | 3.98 ± 1.02 |
| A, B, and C combined (53, 6) | ||||||
| Mean ± SD | 29.2 ± 20.5 | 88.1 ± 92.6 | 36.5 ±21.2 | 18 ± 18.4 | 140.3 ± 174.9 | 49.4 ± 63 |
| Median | 21.2 | 52.4 | 28.2 | 12.3 | 69 | 23.2 |
| Log ± SD | 3.23 ± 0.48 | 4.14 ± 0.79 | 3.46 ± 0.52 | 2.48 ± 0.98 | 4.4 ± 1.03 | 3.33 ± 1.06 |
| Abnormal horses | ||||||
| D (10,9) | ||||||
| Mean ± SD | 30.1 ± 15.4 | 391.0 ± 604.1 | 68.1 ± 45.4 | 47.61 ± 57.4 | 585.1 ± 683.1 | 202.6 ± 231.8 |
| Median | 24.2 | 181 | 62.7 | 15.4 | 215.3 | 43.3 |
| Log ± SD | 3.29 ± 0.49 | 5.19 ± 1.32 | 4.02 ± 0.68 | 3.25 ± 1.16 | 5.61 ± 1.44 | 4.46 ± 1.52 |
| E (25, 15) | ||||||
| Mean ± SD | 106 ± 92.3 (34) | 1,620.1 ± 2,775.5(34) | 360.6 ± 357.5 (38) | 127.0 ± 142.3(17) | 3,640.4 ± 6,589.3 (17) | 778.4 ± 746.2 (18) |
| Median | 71.6 | 972 | 230 | 95.2 | 1,732.5 | 523.0 |
| Log ± SD | 4.34 ± 0.84 | 6.63 ± 1.24 | 5.40 ± 1.06 | 4.26 ±1.19 | 7.17 ± 1.44 | 6.10 ± 1.18 |
| D and E combined (35,24) | ||||||
| Mean ± SD | 81.6 ± 84 (44) | 1,194.7 ± 2,324.3 (44) | 263.1 ± 322.4(48) | 101.5 ± 126.1 (27) | 2,582.8 ± 5,489.5 (26) | 586.5 ± 676.1 (27) |
| Median | 52.4 | 443.5 | 125 | 70.1 | 818.4 | 310.2 |
| Log ± SD | 4.00 ± 0.89 | 6.13 ± 1.43 | 4.94 ± 1.15 | 3.93 ± 1.25 | 6.63 ± 1.60 | 5.56 ±1.5 |
Group A = Clinically normal horses that had no histologic pituitary changes. Group B = Clinically normal non-necropsied horses. Group C = Clinically normal horses with histologic pituitary changes. Group D = Horses suspected to have PPID. Group E = Horses with PPID.
For each group and for all horses, the first number in parentheses is number of horses that had ACTH concentration measured and the second is the number of horses that had α-MSH concentration measured. When the numbers of tests differed from the number of horses, the number of tests is in parentheses at each time period.
Median percentage increase in ACTH concentration (n = 88 horses) and α-MSH concentration (50) 4, 14, and 30 minutes followingTRH administration.
| ACTH | α-MSH | |||||
|---|---|---|---|---|---|---|
| Group | 4 min | 14 min | 30 min | 4 min | 14 min | 30 min |
| A | 68 (23) | 38 (23) | 25 (22) | 335 (15) | 220 (12) | 57 (15) |
| A, B, and C combined | 202 (64) | 44 (55) | 29 (63) | 556 (32) | 228 (17) | 100 (32) |
| E | 825 (34) | 502 (37) | 216 (63) | 1,441 (17) | 840 (7) | 505 (18) |
| D and E combined | 731 (44) | 395 (47) | 190 (48) | 992 (26) | 578 (13) | 384 (27) |
Numbers of tests are in parentheses for each time and hormone. For all groups and times, percentage increase in α-MSH concentration was significantly greater than the percentage increase in ACTH concentration (P < 0.05 for 14 minutes in group E and < 0.01 for all other comparisons).
See Table 1 for remainder of key.
Comparison of the odds ratio, sensitivity, specificity, and percentage of horses with a correct diagnosis of PPID by use of an ACTH concentration ≥ 36 pg/mL and α-MSH concentration ≥ 30 pmol/L and > 50 pmol/L as determined at baseline and 30 minutes after TRH administration for the various groups of horses in Table 1.
| ACTH ≥ 36 pg/mL | α-MSH ≥ 30 < 50 pmol/L | α-MSH > 50 pmol/L | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 min | 30 min | 0 min | 30 min | 0 min | 30 min | |||||||||||||||||||
| Group | OR | S(%) | Sp (%) | CD (%) | OR | S(%) | Sp (%) | CD (%) | OR | S(%) | Sp (%) | CD (%) | OR | S(%) | Sp (%) | CD (%) | OR | S(%) | Sp (%) | CD (%) | OR | S(%) | Sp (%) | CD (%) |
| E vs A | 54 | 71 | 96 | 80 | 180 | 95 | 91 | 93 | 30.3 | 68 | 93 | 79 | — | — | — | — | 24 | 63 | 93 | 76 | — | — | — | — |
| D and E combined vs A | 40 | 65 | 96 | 75 | 70 | 88 | 91 | 89 | 18.7 | 57 | 93 | 70 | 81.2 | 93 | 87 | 90 | 16.2 | 54 | 93 | 67 | 61.6 | 81 | 93 | 86 |
| Evs A, B, and C combined | 15 | 71 | 86 | 80 | 45 | 95 | 71 | 80 | 11.7 | 68 | 84 | 78 | — | — | — | — | 25.7 | 63 | 93 | 82 | — | — | — | — |
| D and E combined vs A, B, and C combined | 11 | 65 | 86 | 77 | 17.5 | 88 | 71 | 78 | 7.2 | 57 | 84 | 72 | 24 | 93 | 66 | 78 | 17.3 | 54 | 94 | 75 | 11.2 | 81 | 72 | 76 |
— = Not applicable. CD = Correct diagnosis for PPID. OR = Odds ratio. S = Sensitivity. Sp = Specificity.
See Table 1 for remainder of key.
The results of the individual TRH tests were examined to determine when ACTH concentration exceeded 36 pg/mL and when α-MSH concentrations exceeded 30 or 50 pmol/L (Table 4). The results were also scrutinized to determine whether ACTH concentrations and α-MSH concentrations diverged and whether high values could be associated with season.
Numbers of individual TRH stimulation tests with ACTH concentration ≥ 36 pg/mL and α-MSH concentrations ≥ 30 and 50 pmol/L at 0 and 30 minutes.
| 0 min | 30 min | |||||
|---|---|---|---|---|---|---|
| Group | α-MSH ≥ 30 < 50 pmol/L | α-MSH > 50 pmol/L | ACTH ≥ 36 pg/mL | α-MSH ≥ 30 < 50 pmol/L | α-MSH > 50 pmol/L | ACTH ≥ 36 pg/mL |
| A | 1/15 | 0/15 | 1/23 | 1/15 | 1/15 | 2/23 |
| B | 1/6 | 1/6 | 1/16(10) | 1/6 (3) | 3/6 (3) | 3/16(10) |
| C | 2/9 | 0/9 | 8/21 (20) | 0/9 | 5/9 | 11/21 (20) |
| A, B, and C combined | 3/30 | 1/30 | 10/60(53) | 2/30 (27) | 9/30 (27) | 16/60 (53) |
| D | 0/9 | 3/9 | 4/10 | 3/9 | 4/9 | 6/10 |
| E | 1/18(15) | 12/18(15) | 26/37 (24) | 0/18 | 18/18(15) | 36/38 (25) |
Number in parentheses is total number of horses if different from number of tests.
See Table 1 for remainder of key.
In the clinically normal horses that had no histologic pituitary changes, for tests in January and July, an ACTH concentration ≥ 36 pg/mL occurred in 1 horse at 0 minutes and in 2 horses at 30 minutes; α-MSH concentration was measured in one of the latter horses and was ≤ 30 pmol/L at both 0 and 30 minutes. At time 0, α-MSH concentration was > 50 pmol/L in 1 clinically normal horse that had no histologic pituitary changes, and at 30 minutes, it was 34 pmol/L in one horse and 114 pmol/L in another; the ACTH concentration was within reference limits in these horses. The α-MSH concentrations > 50 pmol/L were measured in early October.
Of the 8 clinically normal horses with histologic pituitary changes with ACTH concentrations ≥ 36 pg/mL at time 0, 3 also had α-MSH concentration measured and it was never ≥ 30 pmol/L. Only 2 of these 8 tests occurred between August and October. Five of the 11 clinically normal horses with histologic pituitary changes with ACTH concentrations ≥ 36 pg/mL at 30 minutes had α-MSH concentration measured; it was > 90 pmol/L in 3 horses and < 30 pmol/L in 2 horses. At time 0, α-MSH concentrations of 30 pmol/L were found in 2 clinically normal horses with histologic pituitary changes with ACTH concentrations within reference limits. At 30 minutes, 5 clinically normal horses with histologic pituitary changes had α-MSH concentrations > 50 pmol/L and 2 of these horses had ACTH concentrations within reference limits. Concentrations of α-MSH > 30 pmol/L were seen between January and May.
One of 2 clinically normal, non-necropsied horses with baseline ACTH concentrations ≥ 36 pg/mL had α-MSH concentration measured in September, and it was > 90 pmol/L. Another clinically normal, non-necropsied horse with an ACTH concentration within reference limits at time 0 had an α-MSH concentration between 30 and 50 pmol/L in September. Baseline ACTH concentrations increased to ≥ 36 pg/mL at 30 minutes in 5 tests in 3 clinically normal, non-necropsied horses. The tests were performed between mid-July and the end of September. The α-MSH concentration exceeded 50 pmol/L in 2 of these horses and was between 30 and 50 pmol/L in a clinically normal, non-necropsied horse with a normal ACTH concentration at 30 minutes. Two of the high α-MSH concentrations occurred in horses tested in September, 1 in mid-November, and 1 in mid-July. In the PPID group, 15 of 27 tests in which basal ACTH concentration was ≥ 36 pg/mL also had α-MSH concentration measured. The α-MSH concentration was > 50 pmol/L in 12 tests, ≥ 30 but < 50 pmol/L in 1, and < 30 pmol/L in 2. Eighteen of the 33 tests with ACTH concentration ≥ 36 pg/mL at 30 minutes after TRH administration had α-MSH concentration measured; it always was > 50 pmol/L. None of the horses with a diagnosis of PPID with α-MSH concentrations ≥ 30 pmol/L at either time had ACTH concentration < 36 pg/mL. Only 3 of the tests with α-MSH concentration ≥ 30 pmol/L were performed in August through October.
In the horses suspected to have PPID, 3 horses with ACTH concentrations ≥ 36 pg/mL at time 0 had α-MSH concentration measured, and it was > 50 pmol/L. Concentrations of α-MSH > 50 pmol/L were measured in 3 horses at time 0; only 1 had ACTH concentration < 36 pg/mL. At 30 minutes, α-MSH concentration was measured in 5 of the 6 horses with ACTH concentration ≥ 36 pg/mL and was > 30 pmol/L in 1 and > 200 pmol/L in 4. Three of 4 tests where α-MSH concentration was > 50 pmol/L were performed in September.
Domperidone stimulation test—All tests except 1 in September in a horse with PPID were performed between January and June. At baseline and at 2 and 4 hours, ACTH concentrations were significantly (P < 0.05) higher in the PPID group than in the clinically normal horses that had no histologic pituitary changes. At baseline and at 2 and 4 hours, the horses with PPID combined with horses suspected to have PPID had significantly (P < 0.05) higher ACTH concentrations than did clinically normal horses. With an ACTH concentration of ≥ 36 pg/mL defined as abnormal, at baseline, 6 of 8 horses with PPID, 1 of 4 horses suspected to have PPID, 0 of 8 clinically normal horses that had no histologic pituitary changes, 1 of 5 clinically normal horses with histologic pituitary changes, and 0 of 2 clinically normal, non-necropsied horses would be classified as having pituitary dysfunction. At 2 hours, 8 of 8 horses with PPID, 2 of 4 horses suspected to have PPID, 1 of 8 clinically normal horses that had no histologic pituitary changes, 2 of 6 clinically normal horses with histologic pituitary changes, and 0 of 1 clinically normal, non-necropsied horses had ACTH concentrations ≥ 36 pg/mL. At 4 hours, 7 of 8 horses with PPID, 2 of 4 horses suspected to have PPID, 1 of 8 clinically normal horses that had no histologic pituitary changes, 3 of 6 clinically normal horses with histologic pituitary changes, and 0 of 2 clinically normal, non-necropsied horses had ACTH concentrations ≥ 36 pg/mL (Table 5).
Concentration of ACTH (pg/mL) after domperidone administration (2 to 5.5 mg/kg [0.9 to 2.5 mg/lb], PO) to 28 horses.
| Group | 0 min | 2 h | 4 h |
|---|---|---|---|
| A (8) | |||
| Mean ± SD | 14.8 ± 5.4 | 26.5 ± 23.0 | 28.6 ± 20.38 |
| Median | 17 | 20.5 | 22 |
| B(2) | |||
| Mean ± SD | 11.2 ± 2.2 | 20.8* | 18.8 ± 5.7 |
| Median | 11.15 | 20.8 | 18.8 |
| C(6) | |||
| Mean ± SD | 26.3 ± 10.7 | 32.3 ± 10.3 | 37.4 ± 11.4 |
| Median | 24.0 | 31.9 | 37.8 |
| A, B, and C combined (16) | |||
| Mean ± SD | 18.6 ± 9.5 | 28.4 ± 17.8* | 30.7 ± 16.9 |
| Median | 17.9 | 22.5 | 24.6 |
| E(8) | |||
| Mean ± SD | 74.5 ± 52.2 | 253.1 ± 276.9 | 141.9 ± 81.4 |
| Median | 53.4 | 192.5 | 122.5 |
| D(4) | |||
| Mean ± SD | 33.3 ± 17.2 | 83.4 ± 93.6 | 70.3 ± 76.8 |
| Median | 27.9 | 45.7 | 37.9 |
| D and E combined (12) | |||
| Mean ± SD | 60.8 ± 47.3 | 196.5 ± 241.2 | 118.8 ± 84.1 |
| Median | 48.2 | 146 | 103 |
Value missing for 1 horse.
Number of horses and tests per group is in parentheses next to the group letter.
See Table 1 for remainder of key.
Odds ratios, sensitivity, specificity, and percentage of correct diagnoses were determined with an ACTH concentration ≥ 36 pg/mL used to differentiate between all clinically normal horses, clinically normal horses with no histologic pituitary changes, and horses with or suspected to have PPID (Table 6). Prior to administration of domperidone, it was possible only to compare all the clinically normal horses with the clinically abnormal horses, because none of the clinically normal horses that had no histologic pituitary changes had ACTH concentrations ≥ 36 pg/mL. The horses with PPID and horses suspected to have PPID were significantly (P < 0.05) more likely than were the clinically normal group to have basal ACTH concentrations ≥ 36 pg/mL, but the odds ratios and percentage of correct diagnoses for PPID did not increase after administration of domperidone.
Odds ratios, sensitivity, specificity, and percentage of horses with a correct diagnosis of PPID by use of an ACTH concentration ≥ 36 pg/mL in 28 horses that underwent a domperidone challenge test.
| 0 min | 2 h | 4 h | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Group | OR | S (%) | Sp (%) | CD (%) | OR | S (%) | Sp (%) | CD (%) | OR | S (%) | Sp (%) | CD (%) |
| E(8)vsA(8) | — | 75 | — | — | — | — | 89 | — | 49 | 88 | 88 | 88 |
| D and E combined (12) vs A (8) | — | 58 | — | — | 40 | 83 | 89 | 89.5 | 21 | 75 | 88 | 80 |
| E (8) vs A, B, and C combined (16) | 45 | 75 | 94 | 88 | — | — | 81 | — | 21 | 88 | 75 | 79 |
| D and E combined (12) vs A, B, and C combined (16) | 21 | 58 | 94 | 79 | 22 | 83 | 81 | 82 | 9 | 75 | 75 | 75 |
Number of tests or horses in each group are in parentheses.
See Tables 1 and 3 for remainder of key.
The groups were evaluated to determine numbers of horses with a doubling of baseline ACTH concentration (Table 7). When the criterion of doubling of baseline ACTH concentrations was used to determine whether the groups differed at 2 or 4 hours, there were no significant (P > 0.05) differences. The percentage increase in ACTH at 2 and 4 hours did not significantly (P > 0.05) differ between groups. When the groups were examined to determine whether there was a significant difference in the percentage increase in ACTH concentration from baseline, only the clinically normal group had a significant (P < 0.05) percentage increase from baseline at 2 hours. At 4 hours, the horses with PPID, horses suspected to have PPID, clinically normal horses, and clinically normal horses with no histologic pituitary changes had a significant (P < 0.05) percentage increase from baseline. Eleven horses had samples taken at 8 hours (data not shown). Only 1 horse (clinically normal with histologic pituitary changes) had a response different than the response at 2 and 4 hours as there was a ≥ 2-fold increase only at 8 hours.
Numbers of horses with < 2 or ≥ 2-fold increase in ACTH concentration following domperidone administration.
| 2 h | 4 h | |||
|---|---|---|---|---|
| Group | < 2-fold | ≥ 2-fold | < 2-fold | ≥ 2-fold |
| A | 6 | 2 (1) | 5 | 3 (1) |
| C | 5(1) | 1 (1) | 5 (2) | 1 (1) |
| B | 1* | 0 | 2 (0) | — |
| A, B, and C combined | 12 (1) | 3 (2) | 12 (2) | 4 (2) |
| D | 3 (1) | 1 (1) | 3 (1) | 1 (1) |
| E | 5 (5) | 3 (3) | 6 (5) | 2 (2) |
Comparison of the TRH and domperidone stimulation tests—Doubling of baseline ACTH concentration following domperidone administration as a criterion for diagnosis of PPID was compared with the use of ACTH concentration ≥ 36 pg/mL at baseline during the domperidone stimulation test and the use of ACTH concentration ≥ 36 pg/mL at 0 and 30 minutes during the TRH stimulation test in the horses that had had both tests. All of the 8 horses with a diagnosis of PPID had ACTH concentration > 36 pg/mL at 30 minutes after TRH administration, but ACTH increased 2-fold in only 2 of 8 horses at both 2 and 4 hours and in another only at 2 hours after domperidone administration. Responses to the 2 tests agreed in 2 horses suspected to have PPID and diverged in 2 others. In one of the latter horses, baseline ACTH concentration did not increase 2-fold from baseline but did exceed 36 pg/mL at 2 and 4 hours after domperidone administration. Results for both tests were within reference limits in 4 of 8 clinically normal horses that had no histologic pituitary changes. Concentrations of ACTH increased > 2-fold after domperidone administration in 3 clinically normal horses that had no histologic pituitary changes and had normal TRH stimulation test results, although concentration exceeded 36 pg/mL in only one of the domperidone tests. In the clinically normal horses with histologic pituitary changes, results of the 2 tests often diverged; in 3 of the 4 horses in which ACTH concentration did not increase 2-fold after domperidone administration, the response to TRH administration was abnormal. One horse had an abnormal response to domperidone and normal results of TRH testing, results of both tests were normal in one horse, and results of both tests were abnormal in another.
Discussion
The results of the present study indicated that measuring ACTH concentrations, which can be done in commercial laboratories, is at least equal, if not superior, to measuring α-MSH concentrations in diagnosing PPID in horses. When basal ACTH concentrations are higher than the upper reference limit and seasonal influence and disease or stress can be eliminated as confounding factors, there appears to be no advantage to performing either TRH or domperidone stimulation tests to diagnose PPID. However, it is important to ensure samples are obtained with the animal unstressed in its customary environment and there may be variability if only a single baseline sample is assessed. Evaluating ACTH concentration response to TRH administration appeared more accurate in differentiating between normal horses and those with PPID than use of doubling of baseline ACTH concentration following domperidone administration.
A previous study3 reported that horses with PPID had a significantly greater increase in ACTH concentration than did healthy horses after administration of TRH. A cutoff value of 35 pg/mL was used because that is the upper limit of the reference range for the laboratory being used.3 In the present study, we used a value of 36 pg/mL as a more conservative value, which also eliminated dealing with values only fractionally > 35 pg/mL. Forty-eight of the animals in that earlier study are included in the present study, and the sensitivity and specificity for use of the cutoff value of ACTH concentration are similar in both studies for differentiating horses with PPID from clinically normal horses that had no histologic pituitary changes. The present study expanded the number of horses to further evaluate the ACTH concentration response and to compare it with α-MSH concentration changes after TRH administration. As it has been hypothesized that α-MSH concentrations more accurately reflect equine pituitary pars intermedia function, 1 objective of the present study was to determine whether measuring α-MSH concentration after TRH administration would be superior to measuring ACTH concentrations in diagnosing PPID. A previous study showed production of α-MSH but not ACTH in pars intermedia explants from healthy horses when the explants were exposed to TRH, and although plasma concentrations of both α-MSH and ACTH significantly increased in both healthy horses and those with PPID following TRH administration, the α-MSH response was greater.9 More intense immunohistochemical staining for α-MSH than ACTH also has been reported in adenoma cells,16,17 although another study reported the opposite.18 The present study showed that both ACTH and α-MSH concentrations peak prior to 14 minutes after IV administration of 1 mg of TRH, and similar to an earlier study,9 the percentage increase in α-MSH concentration was greater than that of ACTH concentration in both clinically normal horses and those with PPID. As expected, the concentrations of both hormones were higher in the horses with PPID, compared with those in clinically normal horses. Although the percentage increase in α-MSH concentration was often found to be higher than the increase in ACTH concentration at 4 and 14 minutes, there was great individual variability and the increase did not differ between the groups. When the 2 hormones were compared to determine whether use of a set of abnormal values (≥ 36 pg/mL for ACTH and ≥ 30 or 50 pmol/L for α-MSH) at 0 or 30 minutes after TRH administration was superior in differentiating between clinically normal horses and those with PPID, use of α-MSH concentration did not appear superior. For both the ACTH and α-MSH concentrations, specificity was higher than sensitivity at 0 minutes. The percentage of correct diagnoses for use of each hormone value was within 5% except for the comparison between horses with PPID or suspected to have PPID and clinically normal horses that had no histologic pituitary changes, for which use of an α-MSH concentration > 50 pmol/L had 67% correct diagnoses, compared with 75% for ACTH concentration ≥ 36 pg/mL. The reason for the decrease in odds ratio, sensitivity, specificity, and percentage of correct diagnoses when the group of combined clinically normal horses instead of the clinically normal horses with no histologic pituitary changes was compared with the PPID horses could be attributable to the clinically normal group including horses with potential (ie, clinically normal, non-necropsied horses) or confirmed pituitary hyperplasia or adenoma. Although the clinical importance of pituitary changes has been questioned19,20 and inconsistency amongst pathologists in the interpretation of the histologic appearance of the pituitary has been documented,20 the clinically normal horses with histologic pituitary changes had a greater range of values for both ACTH and α-MSH concentrations and many more horses had values above the reference range than did the clinically normal horses that had no histologic pituitary changes, suggesting that histologic pituitary changes may have functional significance and not merely represent morphological changes. However, further studies are needed to establish reference ranges for pituitary cell counts and morphological criteria based on age, sex, and physiologic state and then to correlate findings with basal and evoked POMC secretion. Studies16,18,19 reporting pars intermedia lesions in clinically normal horses have been descriptive and have not reported seasons or endocrine evaluation of the horses. Increased overall mean pituitary weight has been reported in mares without clinical signs, and an age-related increase in lesions (hyperplasia, microadenomas, and adenomas) has been reported to be more prominent in mares than in castrated males and sexually intact males.19 In the present study, sex could not have influenced results in the clinically normal horses with histologic pituitary changes because this group contained slightly more males than females. Interpretation of pituitary changes remains controversial in humans, and functional heterogeneity of morphologically similar POMC cells has been reported in human pituitary glands.21,22 More studies of horses are indicated.
Of interest to veterinarians is the number of horses that individually may be misclassified by a test. In the present study, basal ACTH concentrations ≥ 36 pg/mL were rare in the clinically normal horses that had no histologic pituitary changes but basal concentrations can vary, especially in horses with PPID.3 The results of this study showed similar variability in α-MSH concentrations; when either hormone concentration is low, there is little variation in basal concentration, but high concentrations can differ markedly within 5 to 10 minutes. When basal α-MSH concentration exceeded 100 pmol/L, the 2 values sometimes varied by > 50 pmol/L. It is unclear whether the clinically normal horses with histologic pituitary changes that had ACTH concentration ≥ 36 pg/mL at baseline or 30 minutes had subclinical PPID; ages of the group ranged from 5 to 29 years. Clinically normal horses with histologic pituitary changes with basal ACTH concentrations < 36 pg/mL ranged from 5 to 23 years of age (mean ± SD, 14 ± 6 years; median, 12 years), and those with basal ACTH concentrations ≥ 36 pg/mL ranged from 12 to 29 years of age (20 ± 6 years; median, 18 years), suggesting age and pituitary dysfunction could have influenced values. Normal seasonal increase in α-MSH and ACTH concentrations did not explain the high concentrations in some of the clinically normal horses with histologic pituitary changes; only 2 of the 9 samples with basal ACTH concentration ≥ 36 pg/mL in both tests, 3 of 11 samples with ACTH concentration ≥ 36 pg/mL at 30 minutes after TRH administration, and none of 3 samples with ACTH concentration ≥ 36 pg/mL after domperidone administration were obtained between August and October.
For horses in the PPID group in the present study, use of baseline ACTH concentration alone would have misclassified horses in 11 of 37 TRH stimulation tests and 2 of 8 domperidone stimulation tests. Misclassification decreased to 2 horses when the ACTH concentration at 30 minutes after TRH administration was used. Following domperidone administration, the 2 horses with PPID with normal baseline concentrations had ACTH ≥ 36 pg/mL at 3 of the 4 times it was measured; however, in 1 horse, the concentration increased only to 37.1 pg/mL and did not increase 2-fold, indicating the test result was equivocal. This same horse had a similar response to TRH administration, and it is unknown why neither stimulus evoked the expected increase in ACTH concentration.
When the ACTH response to domperidone administration was compared with that following TRH administration in the same horses, use of the criterion of ≥ 2-fold increase in ACTH concentration from baseline did not appear as accurate in diagnosing PPID as did use of baseline ACTH concentration ≥ 36 pg/mL or ACTH concentration ≥ 36 pg/mL at 30 minutes following TRH administration. A potential reason for the different results in this study compared with earlier reports is difference in case selection for PPID horses. Earlier studies12,13 used hirsutism and a history of improper hair shedding, and animals in those studies may have had more advanced disease than some of the PPID horses in the present study. Variation in domperidone dose was not believed to be a factor as both 1.25 and 5 mg/kg doses of domperidone were reported to differentiate PPID horses from control horses at 2 and 4 hours after administration.b Although the number of horses was small, the results of this study suggest that high baseline ACTH concentrations may be less likely than low baseline values to increase ≥ 2 times after domperidone administration. However, a greater number of horses would have been needed to examine this. It is difficult to adequately compare the data from the present study with previous studies that used 4- and 8-hour sample times, but in this study, in which 11 horses had samples taken at 8 hours, interpretation of the test response was the same as when the 2- and 4-hour responses were used, except in 1 clinically normal horse with histologic pituitary changes. Another potential reason for a difference in domperidone test results among studies is seasonal effect. A previous study12 tested horses between September and January; however, numbers of tests during each month for each group of horses were not specified. If 1 group was tested more frequently in September and October, it is conceivable that results might have been affected. In contrast, in the present study, the domperidone stimulation test was performed between January and June in all horses except 1 with PPID. A drawback to the domperidone stimulation test and a potential reason for variable effects is that the drug is administered by mouth. Poor medication administration technique is unlikely to account for different results, but altered absorption, delayed gastric emptying, the amount of ingesta in the stomach, and individual variation could affect blood concentrations. To our knowledge, there are no published pharmacokinetic data on domperidone in horses. Studiesn in a small number of horses showed blood concentrations appeared to increase linearly with dose, but concentrations varied widely among individuals. There have been no studies indicating the blood concentrations necessary to stimulate ACTH secretion, although 1.1 mg/kg (0.5 mg/lb), PO, is effective in increasing blood concentrations of prolactin.23,24
Domperidone is available in an oral but not parenteral form. Early studies in humans had used forms of domperidone available for IV or IM administration as a prokinetic, but parenteral forms of the drug were subsequently withdrawn because of cardiotoxicity. The oral form appears to have a wide margin of safety in horses and is approved for prophylaxis and treatment of agalactia associated with ingestion of endophyte-infected fescue.23–25 Use of domperidone at a higher dose and as a test for PPID diagnosis would be extralabel use. However, to date, no undesirable effects have been reported in horses, and as it does not typically cross the blood-brain barrier, it is unlikely to have the adverse effects of other dopamine receptor antagonists such as reserpine.23 To our knowledge, no adverse effects attributable to its potential prokinetic activity have been reported in horses. Whether use of an injectable dopamine antagonist such as sulpiride would be diagnostically useful is unknown; it has been given to horses and documented to increase prolactin secretion in mares.24,25
Although TRH is not approved for this application, except for very infrequent minor adverse effects,3 it has been widely used in horses and evaluation of the ACTH response to TRH administration appears useful in diagnosing PPID. Further studies are needed to identify the best tests for diagnosing early or mild PPID.
ABBREVIATIONS
| α-MSH | α-Melanocyte—stimulating hormone |
| POMC | Proopiolipomelanocortin |
| PPID | Pituitary pars intermedia dysfunction |
| TRH | Thyrotropin-releasing hormone |
Miesner TJ, Beard LA, Schmall SM, et al. Results of overnight dexamethasone suppression test repeated over time in horses suspected of having equine Cushing's disease (abstr). J Vet Intern Med 2003;17:420.
Jackson LP, Sojka JE, Moore GE, et al. Dopamine antagonist stimulation test in horses (effect of dose and testing interval on plasma endogenous ACTH) (abstr). J Vet Intern Med 2008;22:708.
Horowitz ML, Neal L, Watson JL. Characteristics of plasma adrenocorticotropin, β-endorphin and α-melanocyte stimulating hormone as diagnostic tests for pituitary pars intermedia dysfunction in the horse (abstr). J Vet Intern Med 2003;17:386.
Terumo Surflo, Terumo Medical Corp, Elkton, Md.
TRH, Sigma-Aldrich Co, St Louis, Mo.
Gibco-Invitrogen Corp, Carlsbad, Calif.
GE Healthcare, Minnetonka, Md.
Domperidone, Equi-Tox, Pharma Inc Central, SC.
Diagnostic Products Corp, Los Angeles, Calif.
Euria α-MSH RIA, American Laboratory Products Co, Windham, NH.
Stata, version 9.2, StataCorp, College Station, Tex.
McFarlane D, Paradis MR, Zimmel D, et al. The effect of geographic location of residence, breed and pituitary dysfunction on seasonal plasma ACTH and α-MSH concentration (abstr), in Proceedings. Dorothy Russell Havemeyer Found Equine Geriatric Workshop 2010;39.
Stewart AJ, Schreiber CM, Behrend EN, et al. Seasonal variation in diagnostic tests for PPID in normal aged geldings (abstr), in Proceedings. Dorothy Russell Havemeyer Found Equine Geriatric Workshop 2010;37.
Longhofer SL, Director, Product Development and Regulatory Affairs, Dechra Pharmaceuticals, Shrewsbury, Shropshire, England: Personal communication, 2009.
References
- 1.
Beech JBoston RCMcFarlane D, et al. Evaluation of plasma ACTH, α-melanocyte-stimulating hormone, and insulin concentrations in clinically normal horses and ponies and those with pituitary pars intermedia dysfunction during different photoperiods. J Am Vet Med Assoc 2009; 235:715–722.
- 2.↑
Donaldson MTMcDonnell SMSchanbacher BJ. Variation in plasma adrenocorticotropic hormone concentration and dexamethasone suppression test results with season, age, and sex in healthy ponies and horses. J Vet Intern Med 2005; 19:217–222.
- 3.↑
Beech JBoston RLindborg S, et al. Adrenocorticotropin concentration following administration of thyrotropin releasing hormone in healthy horses and those with pituitary pars intermedia dysfunction and pituitary hyperplasia. J Am Vet Med Assoc 2007; 23:1–10.
- 4.
Fazio EMedica PFeclazzo A. Seasonal patterns of circulating β endorphin adrenocorticotrophic hormone and cortisol levels in pregnant and barren mares. Bulg J Vet Med 2009; 12:125–135.
- 5.
Dybdal NHargreaves KMMadigan JE. Diagnostic testing for pituitary pars intermedia dysfunction. J Am Vet Med Assoc 1994; 204:627–632.
- 6.
Schott HC IICoursen CLEberhart SW, et al. The Michigan Cushing's Project, in Proceedings. 47th Annu Conv Am Assoc Equine Pract 2001; 22–24.
- 7.↑
Beech JGarcia MC. Hormonal response to thyrotropin releasing hormone in normal horses and those with pituitary adenoma. Am J Vet Res 1985; 46:1941–1943.
- 8.
Thompson JCEllison RGillert R. Problems in the diagnosis of pituitary adenoma (Cushing's syndrome) in horses. N Z Vet J 1995; 43:79–82.
- 9.↑
McFarlane DBeech JCribb A. Alpha-melanocyte stimulating hormone release in response to thyrotopin releasing hormone in healthy horses, horses with pituitary pars intermedia dysfunction and equine pars intermedia explants. Domest Anim Endocrinol 2006; 30:276–288.
- 10.↑
Eiler HOliver JWAndrews FM, et al. Results of a combined dexamethasone suppression/thyrotropin releasing hormone stimulation test in healthy horses and horses suspected to have a pars intermedia pituitary adenoma. J Am Vet Med Assoc 1997; 211:79–81.
- 11.↑
Frank NAndrews FMSommardahl CS, et al. Evaluation of the combined dexamethasone suppression/thryrotropin-releasing hormone stimulation test for detection of pars intermedia adenomas in horses. J Vet Intern Med 2006; 20:987–993.
- 12.↑
Sojka JEJackson LPMoore G, et al. Domperidone causes an increase in endogenous ACTH concentration in horses with pituitary pars intermedia dysfunction (equine Cushings' disease), in Proceedings. 52nd Annu Conv Am Assoc Equine Pract 2006; 320–323.
- 13.
Miller MAPardo IDJackson GEM, et al. Correlation of pituitary histomorphometry with adrenocorticotrophic hormone response to domperidone administration in the diagnosis of equine pituitary pars intermedia dysfunction. Vet Pathol 2008; 45:26–28.
- 14.
McFarlane DDonaldson MTMcDonnell SM, et al. Effects of season and sample handling on measurement of plasma α melanocyte-stimulating hormone concentrations in horses and ponies. Am J Vet Res 2004; 65:1463–1468.
- 15.
McFarlane DHolbrook TC. Cytokine dysregulation in aged horses and horses with pituitary pars intermedia dysfunction. J Vet Intern Med 2008; 22:436–442.
- 16.
Boujon CEBestetti GEMeier HP, et al. Equine pituitary adenoma: a functional and morphological study. J Comp Pathol 1993; 109:163–178.
- 17.
Henrichs MBaumgartner WCapen CC. Immunocytochemical demonstration of proopiomelanocortin derived peptides in pituitary adenomas of the pars intermedia of horses. Vet Pathol 1990; 27:419–425.
- 18.↑
Yoshikawa HOishi HSumi A, et al. Spontaneous pituitary adenomas of the pars intermedia in 5 aged horses: histopathological immunohistochemical and ultrastructural studies. J Equine Sci 2001; 12:119–126.
- 19.↑
Van der Kolk JHHeinricks Mvan Amerongen JD, et al. Evaluation of pituitary gland anatomy and histopathologic findings in clinically normal horses and horses and ponies with pituitary pars intermedia adenoma. Am J Vet Res 2004; 65:1701–1707.
- 20.↑
McFarlane DMiller LMCraig LE, et al. Agreement in histologic assessments of the pituitary pars intermedia of aged horses. Am J Vet Res 2005; 66:2055–2059.
- 21.
Daly AFBurlacu MDLivadariu, et al. The epidemiology and management of pituitary incidentalomas. Horm Res 2007; 68:195–298.
- 22.
Horvath EKovacs KLloyd RV. Pars intermedia of the human pituitary revisited: morphologic aspects and frequency of hyperplasia of POMC-peptide immunoreactive cells. Endocr Pathol 1999; 10:55–64.
- 23.↑
Evans TJYoungquist RSLoch WE, et al. A comparison of the relative efficacies of domperidone and reserpine in treating Equine “fescue toxicosis,” in Proceedings. 45th Annu Meet Am Assoc Equine Pract 1999; 207–209.
- 24.
Redmond LMCross DLStrickland JR, et al. Efficacy of domperidone and sulpiride as treatments for fescue toxicosis in horses. Am J Vet Res 1994; 55:722–729.
- 25.
Guillaume DChavatte-Palmer PCombarnous Y, et al. Induced lactation with a dopamine antagonist in mares: different responses between ovariectomized and intact mares. Reprod Dom Anim 2003; 38:394–400.
