Alpha-melanocyte stimulating hormone release in response to thyrotropin releasing hormone in healthy horses, horses with pituitary pars intermedia dysfunction and equine pars intermedia explants

Metabolic and Endocrine Insights in Donkeys

Donkey medicine is gaining attention due to their increased use as companion animals, in shows, asinotherapy, etc. The increasing demand and unique aspects call for specialized care, requiring new information (physiology, infectious disorders, pharmacology, etc.). Since obesity is common in this species, hyperlipemia, metabolic syndrome and insulin dysregulation (ID) are common disorders in donkeys, in some cases with high mortality, either directly (multiorgan dysfunction) or indirectly due to poor quality of life (chronic laminitis). Donkeys have long-life expectancy and are often afflicted with pituitary pars intermedia dysfunction (PPID), a neurodegenerative and endocrine disease. Hyperlipemia is diagnosed based on high plasma triglyceride concentration in association with clinical findings and laboratory abnormalities from affected tissues (liver, kidney and pancreas). The measurement of resting serum insulin and plasma ACTH concentrations is the first step in ID and PPID diagnosis. In donkeys with clinical signs of ID (obesity or recurrent laminitis) or PPID (hypertrichosis, regional adiposity, laminitis and weight loss), where these hormones are in the normal or non-diagnostic range (donkey-specific cut-off values and reference ranges need to be established), dynamic tests are recommended (oral sugar test or thyrotropin-releasing hormone, respectively). Equine treatment protocols apply to donkeys, although pharmacological studies for most drugs, except pergolide, are lacking.

Evaluation of seasonal influences on adrenocorticotropic hormone response to the thyrotropin-releasing hormone stimulation test and its accuracy for diagnosis of pituitary pars intermedia dysfunction

Pituitary pars intermedia dysfunction (PPID) is an age-related neurodegenerative disorder, affecting >20 % of older horses. There is a need for improved endocrine tests for early disease detection, and the thyrotropin-releasing hormone (TRH) stimulation test has been recommended for diagnosis of early or mild cases. However, it is currently not recommended for year-round use due to marked seasonal variability. The aims of this cohort study were to evaluate effects of month and season on adrenocorticotropic hormone (ACTH) responses to TRH stimulation and to derive monthly cut-offs for PPID diagnosis. Sixty-three horses were assigned to control (n = 17), subclinical PPID (n = 21) and clinical PPID (n = 25) groups, based on a composite reference standard that combined clinical history and examination findings with endocrine test results. TRH stimulation tests were performed monthly for a 12-month period. Circannual changes were evaluated with one- and two-way repeated-measures analysis of variance and receiver operating characteristic curve analysis was used to derive cut-off values for basal and TRH-stimulated ACTH. TRH-stimulated ACTH concentrations were lowest in February-May and highest in August-October. Specificity of both basal and 30 min post-TRH ACTH was generally higher than sensitivity, and TRH stimulation had improved diagnostic accuracy compared to basal ACTH, although its sensitivity was not significantly greater year-round. TRH stimulation tests yielded considerably more positive results than basal ACTH in the subclinical group, but few additional positive results in clinical PPID cases. There were large differences between cut-offs that maximised sensitivity or specificity for TRH-stimulated ACTH, highlighting the importance of considering clinical presentation alongside test results in diagnostic decision-making.

Diagnosis of equine pituitary pars intermedia dysfunction

Equine pituitary pars intermedia dysfunction (PPID) is common in aged horses. The majority of horses respond well to treatment, but treatment is lifelong, meaning accurate diagnosis of PPID is important. Similar to any condition, there is no perfect laboratory test to diagnose PPID and accuracy is affected by the characteristics of the population in which the test is being evaluated. This review details the importance of consideration of clinical factors and diagnostic test accuracy. Basal adrenocorticotrophic hormone (ACTH) concentration is used most frequently in practice and has very good diagnostic accuracy when used in combination with clinical judgement and the correct application of diagnostic thresholds. The thyrotropin-releasing hormone stimulation test can be used in horses with equivocal test results following basal ACTH testing, or to evaluate subtle cases due to its improved accuracy.

Prospective Case Series of Clinical Signs and Adrenocorticotrophin (ACTH) Concentrations in Seven Horses Transitioning to Pituitary Pars Intermedia Dysfunction (PPID)

Simple Summary

Pituitary pars intermedia dysfunction (PPID) is a common disease of the geriatric horse population. The most common clinical sign of PPID is hypertrichosis, or a long hair-coat with delayed shedding. Hypertrichosis is the most easily recognized clinical sign of PPID. However, the presence of hypertrichosis is often associated with severe end-stage disease. There is little research investigating sub-clinical or early PPID and the clinical signs associated with these stages of disease. The benefit of being able to recognize early PPID, is that we are able to begin treatment earlier on in disease process, potentially reducing the deleterious consequences of PPID and improving survival. Laboratory tests are available to more accurately diagnose PPID, and these tests include the basal ACTH and TRH-stimulated ACTH tests. Basal ACTH is easy to perform and is recommended in cases where clinical disease is suspected. The TRH-stimulation test improves diagnostic accuracy in early PPID cases. This study documents both test results and clinical signs associated with the transition from subclinical to clinical PPID, so that we are better able to recognize potential early PPID, as well as interpret results in these horses.

Abstract

Poor recognition of subtle clinical abnormalities and equivocal ACTH concentrations make early diagnosis of PPID difficult. Progressive clinical findings and corresponding ACTH concentrations in horses transitioning to PPID over time have not been documented. Seven horses with ACTH concentrations equivocal for PPID (utilizing locally derived, seasonally adjusted diagnostic-cut off values (DCOV)) and no clinical signs of PPID were selected. Sequential measurement of basal and thyrotropin-releasing hormone (TRH)-stimulated ACTH concentrations and recording of clinical findings occurred from October 2017 to November 2021 in a prospective case series. In two horses, marked hypertrichosis developed. Although 1/11 basal ACTH concentrations were below DCOV in 2018, subsequently all basal ACTH concentrations in these two horses without treatment were greater than DCOV. One horse was treated with pergolide which normalized basal ACTH concentrations. Four horses developed intermittent, mild hypertrichosis, and one horse never developed hypertrichosis. Basal ACTH concentrations in these five horses were greater than DCOV in 63/133 (47.4%) of testing points. TRH-stimulated ACTH concentrations in these five horses were greater than DCOV in 77/133 (57.9%) of testing points, sometimes markedly increased and greater than the assay upper limit of detection (LoD) of 1250pg/mL. TRH-stimulated ACTH concentrations were most frequently positive in late summer and early autumn, with 24/37 (64.9%) of TRH-stimulated ACTH concentrations greater than the DCOV in February and March. Horses transitioning to PPID can have subtle clinical signs and equivocal ACTH concentrations. However, TRH-stimulated ACTH concentrations can be markedly greater than DCOV, especially in late summer and early autumn (February and March) allowing for identification of subclinical and transitional cases.

Pituitary Pars Intermedia Dysfunction (PPID) in Horses

Substantial morbidity results from pituitary pars intermedia dysfunction (PPID) which is often underestimated by owners and veterinarians. Clinical signs, pathophysiology, diagnostic tests, and treatment protocols of this condition are reviewed. The importance of improved recognition of early clinical signs and diagnosis are highlighted, as initiation of treatment will result in improved quality of life. Future research should be targeted at improving the accuracy of the diagnosis of PPID, as basal adrenocorticotropic hormone (ACTH) concentration can lack sensitivity and thyrotropin releasing hormone (TRH) used to assess ACTH response to TRH stimulation is not commercially available as a sterile registered product in many countries. The relationship between PPID and insulin dysregulation and its association with laminitis, as well as additional management practices and long-term responses to treatment with pergolide also require further investigation.

The effect of pergolide mesylate on adrenocorticotrophic hormone responses to exogenous thyrotropin releasing hormone in horses

Thyrotropin releasing hormone (TRH) stimulation testing is often used to support a diagnosis of pituitary pars intermedia dysfunction (PPID) in horses although it is unclear whether or not repeat TRH stimulation testing post-treatment is a valid means of assessing response to medical therapy. Laboratory submissions from 64 suspected equine PPID cases were examined including the initial pre-treatment TRH stimulation test and a follow up test within 100 days of starting medical therapy with pergolide. In a subset of cases, further follow-up tests were examined beyond 100 days of starting treatment. Results from tests conducted between 1 July and 30 November 30 were excluded. Significant improvements were seen in both the baseline and TRH-stimulated adrenocorticotrophic hormone (ACTH) concentrations within 100 days with no further improvements seen in the subset of cases examined thereafter. Although 88% (n=56/64) of all cases showed a decreased response to TRH post-treatment, only 24% (n=9/38) of horses with positive pre-treatment TRH stimulation tests normalised following treatment, with a further 34% (n=13/38) improving into an equivocal test outcome category. Most commonly (42%; n=16/38), horses with positive pre-treatment TRH stimulation tests remained positive following treatment, although 75% (n=12/16) of these showed a numerically lower post-treatment response to TRH. These results will help inform practitioners of expected changes in TRH stimulation test results when assessing response of horses with PPID to medical therapy with pergolide.

Effect of early or late blood sampling on thyrotropin releasing hormone stimulation test results in horses

Background

Diagnosis of pituitary pars intermedia dysfunction (PPID) using the thyrotropin‐releasing hormone (TRH) stimulation test requires blood collection 10 minutes after TRH injection; it is unknown if small differences in timing affect test results.

Objective

To determine whether early or late sampling results in a significant (≥10%) difference in plasma adrenocorticotropic hormone (ACTH) concentration compared to standard 10‐minute sampling.

Animals

Twenty‐four healthy adult horses with unknown PPID status.

Methods

In this prospective study, subjects underwent a single TRH stimulation test, with blood collected exactly 9 minutes (early), 10 minutes (standard), and 11 minutes (late) after injection. ACTH was measured by chemiluminescent immunoassay. Two aliquots of the 10‐minute plasma sample were analyzed separately to assess intra‐assay variability. Data were reported descriptively and bias was calculated using Bland‐Altman plots. Significance was set at P = .05.

Results

Minor variability was observed between the paired 10‐minute sample aliquots (range, 0%‐6%; median 3%). Overall variability of early or late samples compared to the corresponding paired (average) 10‐minute standard concentration ranged from 0% to 92% (median 10%). Seventy‐five percent of horses (18/24) tested had at least 1 early or late reading that differed by ≥10% from its corresponding 10‐minute standard concentration, and 21% of horses (5/24) would have a different interpretation of testing result with either early or late sampling. Incidence of ≥10% variability was independent of PPID status (P = .59).

Conclusions and Clinical Importance

Precise timing of sample collection is critical to ensure accurate assessment of PPID status given the observation of significant variability associated with minor alterations in timing of sample collection.

Repeatability of a thyrotropin-releasing hormone stimulation test for diagnosis of pituitary pars intermedia dysfunction in mature horses

BACKGROUND

Pituitary pars intermedia dysfunction (PPID) is a common endocrinopathy of horses diagnosed with a thyrotropin-releasing hormone (TRH) stimulation test.

HYPOTHESIS/OBJECTIVES

Describe the repeatability of TRH stimulation in horses with and without PPID in winter and autumn.

ANIMALS

Twenty adult horses; 6 controls and 6 with PPID tested in autumn, 8 controls and 6 with PPID tested in winter with 3 controls and 3 with PPID tested in both seasons.

METHODS

Thyrotropin-releasing hormone stimulation was performed on 2 consecutive occasions, 1 week before and 1 week after the winter solstice and the autumn equinox. Blood was collected before and 30 minutes after IV injection of 1 mg of TRH. ACTH concentration was determined by a chemiluminescent assay. Repeatability and test-retest reliability were assessed by repeated measures analysis of variance, intraclass correlation coefficient and within-horse coefficients of variation (CV). Bland-Altman plots were generated to visualize agreement between repetitions.

RESULTS

In winter, no week effect was detected on the results of the TRH simulation and the test had an excellent test-retest reliability. In autumn, after-TRH ACTH concentrations were significantly lower on week 2 (P = .02) and the test only had a good test-retest reliability. There were significantly larger within-horse CV during autumn (P = .04) and after TRH stimulation (P = .04). There were 2 misclassifications in winter and 4 in autumn.

CONCLUSIONS AND CLINICAL IMPORTANCE

The TRH stimulation test was repeatable when performed 2 weeks apart in winter; however, in autumn, more variability in after-TRH ACTH concentrations resulted in decreased repeatability.

Evaluation of the sensitivity and specificity of basal plasma adrenocorticotrophic hormone concentration for diagnosing pituitary pars intermedia dysfunction in horses: A systematic review

Measurement of basal adrenocorticotrophic hormone (ACTH) is currently used to diagnose pituitary pars intermedia dysfunction (PPID) in horses, yet a systematic review of the evidence for its use has not been undertaken. This study aimed to systematically review evidence regarding the sensitivity and specificity of the basal ACTH diagnostic test. Electronic databases were systematically searched in January 2019, September 2020 and January 2021, for English language publications published prior to these dates. Screening, data extraction and quality assessment of publications was undertaken by the authors using predefined criteria. Study design, methodology and information reported in included studies were assessed using Standards for Reporting of Diagnostic Accuracy (STARD) checklists. Risk of bias and applicability were appraised using the Quality Assessment tool for Diagnostic Accuracy Studies (QUADAS-2) quality assessment tool. Due to identified biases and marked between-study variations, meta-analysis was not undertaken. After removal of duplicates, 415 publications were identified, of which 25 were evaluated in full, with 11 of these meeting inclusion criteria. In most studies, basal ACTH was reported to have good sensitivity (overall median 75.5%; interquartile range [IQR], 64.0-86.5%; range, 36.0-100%) and excellent specificity (overall median, 95.2%; IQR, 84.2-98.9%; range, 63.3-100%). However, QUADAS-2 and STARD assessment highlighted that studies did not utilise optimal study design and/or study populations for the evaluation of a diagnostic test and the majority were subject to bias, or provided insufficient information to fully assess possible biases. Based on this review, basal ACTH performed better at ruling out PPID than detecting it.

Plasma prolactin, thyroid-stimulating hormone, melanocyte-stimulating hormone, and adrenocorticotropin responses to thyrotropin-releasing hormone in mares treated with detomidine and butorphanol

Stress or excitement is a concern when performing endocrine tests on fractious horses. Sedation may be a solution; however, perturbation of test results may preclude useful information. Thyrotropin-releasing hormone (TRH) is a known stimulator of prolactin, thyroid-stimulating hormone (TSH), melanocyte-stimulating hormone (MSH), and ACTH. Thyrotropin-releasing hormone-induced ACTH is a diagnostic tool for the assessment of endocrinopathies such as pituitary pars intermedia dysfunction. It is unknown if drugs commonly used for sedation alter endocrine responses. The objective of this study was to assess the effects of detomidine (DET) and butorphanol on endocrine responses to TRH. Nine light horse mares were used in a replicated 3 × 3 Latin square with the following treatments: saline, DET, and detomidine + butorphanol (DET/BUT), all administered intravenously at 0.01 mg/kg BW. A 1-wk washout period was allowed between phases, all of which were performed in December. Blood samples were collected at -10 and 0 min before treatment and 5 and 10 min post-treatment. Administration of 1 mg TRH occurred 10 min post-treatment, and blood sampling continued 5, 10, 20, and 30 min post-TRH. Data were analyzed by ANOVA as a replicated Latin square with repeated sampling. Plasma prolactin increased (P < 0.0001) after TRH in all groups, rapidly peaking at 5 min in drug-treated mares and 40 min in saline-treated mares. The peak prolactin response to TRH was 2-fold higher (P < 0.0001) in saline-treated mares compared with those drug-treated. A peak rise in plasma TSH was observed in DET/BUT-treated mares 10 min after TSH and was greater (P ≤ 0.007) compared with DET- and saline-treated mares. Plasma MSH was stimulated (P = 0.001) by DET and DET/BUT before TRH, and the peak MSH response to TRH was greater (P < 0.0001) in drug-treated mares, although not hastened as observed with prolactin and TSH. A peak rise in ACTH was observed in drug-treated mares 5 min after administration of TRH, whereas a peak rise was observed in control mares 10 min post-TRH and was almost 2-fold lower (P = 0.05) than the peak observed in DET and DET/BUT-treated mares. Basal ACTH concentrations were not affected by DET or DET/BUT, indicating that sedation with these compounds may be achieved when needing to measure basal plasma ACTH. Treatment with DET and DET/BUT did alter the prolactin, TSH, MSH, and ACTH responses to TRH; therefore, the use of these drugs may not be advisable when assessing endocrine responses to TRH stimulation.

Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses

BACKGROUND

Diagnosis of pituitary pars intermedia dysfunction (PPID) is problematic because of large variations in ACTH concentrations.

HYPOTHESIS/OBJECTIVES

Compare the test characteristics of baseline and post-thyrotropin-releasing hormone (TRH) stimulation plasma ACTH concentrations in horses using diagnostic cutoff values (DCOVs) and reference intervals (RIs) and determine the clinical consequences of using each method.

ANIMALS

One hundred six mature horses: 72 control cases and 34 PPID cases.

METHODS

Prospective case-controlled study. Horses underwent monthly TRH stimulation tests. Diagnostic cutoff values were determined monthly by receiver operating characteristic curves using the Youden index. Reference intervals were determined monthly by a robust method. For each case age, sex and body condition score (BCS) were recorded.

RESULTS

Baseline ACTH concentrations varied by month (P < .001) with significant "month × age" (P = .003), "month × sex" (P = .003), and "month × BCS" (P = .007) effects. Baseline ACTH concentrations were accurate to diagnose PPID (0.91 ± 0.06) with DCOVs increasing the test sensitivity (0.61 ± 0.21 to 0.87 ± 0.05, P = .002) and RI increasing test specificity (0.85 ± 0.12 to 0.98 ± 0.01, P = .01). Thyrotropin-releasing hormone stimulation improved test accuracy (0.91 ± 0.06 to 0.97 ± 0.03, P = .004).

CONCLUSIONS AND CLINICAL IMPORTANCE

ACTH concentrations follow a circannual rhythm and vary with physiological factors. As using DCOVs increases the ability to detect mild cases and using RI decreases the risk of unnecessary treatments, ACTH concentrations should be interpreted within a specific clinical context. The TRH stimulation test improves the diagnosis of PPID.

Efficacy of pergolide for the management of equine pituitary pars intermedia dysfunction: A systematic review

Pergolide, a dopamine agonist, is commonly administered to manage pituitary pars intermedia dysfunction (PPID), a progressive neurodegenerative disease prevalent in aged horses. However, available evidence regarding pergolide's efficacy in improving clinical and endocrine parameters is limited. The aim of this systematic review was to assess published literature and evaluate evidence regarding whether pergolide treatment results in improvement of clinical signs and/or adrenocorticotrophic hormone (ACTH) concentration compared to no treatment or other unlicensed treatments. Systematic searches of electronic databases were undertaken in April 2019, repeated in August and October 2019, and updated in July 2020. English language publications published prior to these dates were included. Screening, data extraction and quality assessment of publications was undertaken individually by the authors using predefined criteria and subsequently cross-checked. Modified critically appraised topic data collection forms were used to extract data. Due to marked between-study variations, meta-analysis was not undertaken. After removal of duplicate records; 612 publications were identified, of which 129 abstracts were screened for eligibility and 28 publications met criteria for inclusion in the review. Most studies were descriptive case series, cohort studies or non-randomised, uncontrolled field trials. Despite marked variation in study populations, case selection, diagnostic protocols, pergolide dose, follow-up period and outcome measures, in the vast majority of the included studies, pergolide was reported to provide overall clinical improvement in >75% of cases. However, reported improvements in individual clinical signs varied widely. A reduction in plasma ACTH concentrations was reported in 44-74% of cases, while normalisation to within reported reference intervals occurred in 28-74% of cases.

Can Endocrine Dysfunction Be Reliably Tested in Aged Horses That Are Experiencing Pain?

Simple Summary

Pituitary pars intermedia dysfunction (PPID) is an endocrine (secreting internally) disease of aged horses and ponies. An enlargement (hyperplasia) of the pars intermedia of the pituitary gland leads to an increased secretion of hormones, including adrenocorticotropic hormone (ACTH). The main tests for a diagnosis of PPID are the measurement of basal ACTH and the thyrotropin-releasing hormone (TRH) stimulation test, where TRH stimulates the secretion of ACTH. Since pain can also lead to elevated concentrations of ACTH, it is unclear whether horses with pain can be tested for PPID correctly. The aim of the present study was to find out whether pain caused a marked increase of ACTH can lead to a false positive result in the diagnosis of PPID. Therefore, we examined fifteen horses treated for different pain conditions, which also served as their own controls as soon as they were pain-free again. The ACTH and cortisol were measured before and after the TRH stimulation test. There was no significant difference in the ACTH concentration in horses with pain and the controls, between different pain intensities or between disease groups. Thus, measuring the basal ACTH concentration and performing the TRH stimulation test for the diagnosis of PPID seem to be possible in horses with a treated low to moderate pain condition.

Abstract

The aim of the present study was to evaluate (i) the effects of different intensities and types of treated pain on the basal concentrations of adrenocorticotropic hormone (ACTH) and cortisol, and (ii) the thyrotropin-releasing hormone (TRH) stimulation test, to determine whether treated pain caused a marked increase of ACTH, which would lead to a false positive result in the diagnosis of pituitary pars intermedia dysfunction (PPID). Methods: Fifteen horses with treated low to moderate pain intensities were part of the study. They served as their own controls as soon as they were pain-free again. The horses were divided into three disease groups, depending on their underlying disease (disease group 1 = colic, disease group 2 = laminitis, disease group 3 = orthopedic problems). A composite pain scale was used to evaluate the intensity of the pain. This pain scale contained a general part and specific criteria for every disease. Subsequently, ACTH and cortisol were measured before and after the intravenous application of 1 mg of TRH. Results: There was no significant difference in the basal or stimulated ACTH concentration in horses with pain and controls, between different pain intensities or between disease groups. Descriptive statistics, however, revealed that pain might decrease the effect of TRH on the secretion of ACTH. There was an increase of ACTH 30 min after TRH application (p = 0.007) in the treated pain group, but this difference could not be statistically confirmed. Measuring the basal ACTH concentration and performing the TRH stimulation test for the diagnosis of PPID seem to be possible in horses with low to moderate pain.

Clinically and temporally specific diagnostic thresholds for plasma ACTH in the horse

BACKGROUND

Pituitary pars intermedia dysfunction (PPID) is commonly investigated using plasma ACTH concentrations but problems exist with currently available diagnostic thresholds.

OBJECTIVES

To derive temporally specific diagnostic thresholds for equine plasma ACTH concentration to be used alongside clinical judgement in each individual week of the year and appropriate for the degree of clinical suspicion in any given case. Furthermore, to apply these thresholds to compare the prevalence of high and low ACTH in two subgroups of animals with high and low clinical suspicion of PPID.

STUDY DESIGN

A retrospective population study examining a large laboratory database of equine plasma ACTH concentrations using an indirect approach to calculate diagnostic thresholds.

METHODS

Logs of plasma ACTH concentrations from 75 892 individual horses were examined using robust L₂ estimation of mixtures of two normal distributions in categories of each week and month of the year. Thresholds dividing the two populations of high-ACTH and low-ACTH horses were then established at different levels of sensitivity and specificity and compared with clinical subgroups of horses divided based on reported clinical signs, as having high (n = 4036) or low (n = 3022) clinical suspicion of PPID.

RESULTS

For most of the year there were small interweek differences in diagnostic thresholds. However, from mid-June to early-December diagnostic thresholds showed greater interweek variability, reaching a maximum in late September and early October. Grouping of high- and low-ACTH compared favourably with grouping based on clinical signs.

MAIN LIMITATIONS

Given the multiple sources of diagnostic samples, pre-analytical data could not be fully verified.

CONCLUSIONS

Diagnostic thresholds for equine plasma ACTH vary through the year. It is especially important to consider the temporally specific threshold between June and December. Different clinical thresholds can be used depending on the case circumstances and whether a false-positive or false-negative diagnosis is deemed least desirable.

The effect of freeze-thaw cycles on determination of immunoreactive plasma adrenocorticotrophic hormone concentrations in horses

Background

Determination of plasma adrenocotrophic hormone (ACTH) concentration (endogenous or thyrotropin‐releasing hormone [TRH] stimulation test) is the most commonly used diagnostic test for pituitary pars intermedia dysfunction (PPID) in horses. Because ACTH is unstable, samples often are frozen to be shipped to laboratories or to allow for batch analysis of research samples. However, the effect of multiple freeze‐thaw cycles on equine ACTH is unknown.

Objective

To determine the effects of multiple freeze‐thaw cycles on immunoreactive ACTH concentration.

Animals

Twenty‐eight horses ranging from 10 to 27 years of age were used.

Methods

Prospective study. Horses were divided into 4 groups: group 1, PPID‐negative, without TRH stimulation; group 2, PPID‐negative, with TRH stimulation; group 3, PPID‐positive, without TRH stimulation; and group 4, PPID‐positive, with TRH stimulation. Whole blood was collected from each horse at baseline or 30 minutes after TRH stimulation. Immunoreactive plasma ACTH concentration was determined using a chemiluminescence assay. Plasma samples then were frozen at −80°C >24 hours, thawed at 4°C and reanalyzed for 5 freeze‐thaw cycles. Changes in plasma ACTH concentration were analyzed using a linear mixed‐effect model.

Results

Significant effects of freeze‐thaw cycles (P = .001) and PPID status (P = .04) on plasma ACTH concentration were observed, but no significant effect of TRH stimulation was identified.

Conclusions and Clinical Importance

The plasma ACTH concentration is altered by freeze‐thaw cycles, and the effect is observed sooner in horses with PPID. To diagnose PPID, multiple freeze‐thaw cycles should be avoided when measuring plasma ACTH concentration.

Relationships of inflamm-aging with circulating nutrient levels, body composition, age, and pituitary pars intermedia dysfunction in a senior horse population

Similarly to aged humans, senior horses (≥20 years) exhibit chronic low-grade inflammation systemically, known as inflamm-aging. Inflamm-aging in the senior horse has been characterized by increased circulating inflammatory cytokines as well as increased inflammatory cytokine production by lymphocytes and monocytes in response to a mitogen. Little is currently known regarding underlying causes of inflamm-aging. However, senior horses are also known to present with muscle wasting and often the endocrinopathy pituitary pars intermedia dysfunction (PPID). Despite the concurrence of these phenomena, the relationships inflamm-aging may have with measures of body composition and pituitary function in the horse remain unknown. Furthermore, nutrition has been a focus of research in an attempt to promote health span as well as life span in senior horses, with some nutrients, such as omega-3 fatty acids, having known anti-inflammatory effects. Thus, an exploratory study of a population of n = 42 similarly-managed senior horses was conducted to determine relationships between inflamm-aging and measures of circulating nutrients, body composition, age, and PPID. Serum was collected to determine vitamin, mineral, and fatty acid content. Peripheral blood mononuclear cells were also isolated to determine inflammatory cytokine production of interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) following stimulation with a mitogen, as well as to determine gene expression of interleukin(IL)-1β, IL-6, IL-10, IFN-γ, and TNF-α. Serum IL-6 and C-reactive protein were determined by enzyme-linked immunosorbent assay. Whole blood was collected for hematological and biochemical analysis. Body composition was evaluated via ultrasound and muscle scoring for all 42 horses as well as by deuterium oxide dilution for a subset of n = 10 horses. Pituitary function was evaluated by measuring basal adrenocorticotropin hormone concentrations as well as by thyrotropin releasing hormone stimulation testing (to determine PPID status). Results showed various relationships between inflammatory markers and the other variables measured. Most notably, docosadienoic acid (C22:2n6c), docosapentaenoic acid (C22:5n3c), and folate were positively associated with numerous inflammatory parameters (P ≤ 0.05). Although no relationships were found between inflamm-aging and PPID, being positive for PPID was negatively associated with vitamin B12 (P ≤ 0.01). No relationships between inflammation and body composition were found. Even within this senior horse population, age was associated with multiple parameters, particularly with numerous inflammatory cytokines and fatty acids. In summary, inflamm-aging exhibited relationships with various other parameters examined, particularly with certain fatty acids. This exploratory study provides insights into physiological changes associated with inflamm-aging in the senior horse.

Effects of Docosahexaenoic Acid-Rich Microalgae Supplementation on Metabolic and Inflammatory Parameters in Horses With Equine Metabolic Syndrome

Much of the equine population is obese and therefore predisposed to the development of additional health concerns such as equine metabolic syndrome (EMS). However, pharmacologic treatments for EMS are limited. Omega-3 fatty acid supplementation is a therapeutic strategy in humans with metabolic dysfunction that improves insulin sensitivity and reduces inflammation, but the effects of omega-3 fatty acid supplementation in horses with EMS are unclear. Therefore, in this pilot study, 10 mixed-sex and mixed-breed horses with EMS were fed a docosahexaenoic acid (DHA)-rich microalgae containing 16 g DHA/horse/d or served as controls for 46 days. Inflammatory status was measured using serologic enzyme-linked immunosorbent assay and in peripheral blood mononuclear cells (PBMCs) using flow cytometry and reverse transcription polymerase chain reaction. Circulating fatty acids, triglyceride, leptin, and adiponectin concentrations were also determined. Insulin and glucose dynamics were assessed with oral sugar test (OST) and frequently sampled intravenous glucose tolerance testing. Postsupplementation, treated horses had an increase in many circulating fatty acids, including DHA (P < .001). Treated horses also had lower serum triglycerides postsupplementation (P = .02) and a trend (P = .07) for reduced PBMC tumor necrosis factor α. Interestingly, after 46 days, control horses had an increase in insulin responses to the OST (P = .01), whereas treated horses did not (P = .69). These pilot data indicate that DHA-rich microalgae supplementation alters circulating fatty acids, modulates metabolic parameters, and may reduce inflammation in horses with EMS.

Diagnostic Testing for Equine Endocrine Diseases: Confirmation Versus Confusion

Despite there being only 2 common endocrine diseases in horses, pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS), diagnosis is still confusing. Failing to consider horse factors and treating based on laboratory results only have caused many animals to receive lifelong drug treatment unnecessarily. Increased plasma ACTH, baseline or TRH stimulated, supports a diagnosis of PPID; however, breed, age, thriftiness, illness, coat color, geography, diet, and season also affect ACTH concentration. Insulin dysregulation, the hallmark of EMS, can result from insulin resistance or excessive postprandial insulin release. Each requires a different diagnostic test to reach a diagnosis.

Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin-Stimulating Hormone in Horses with PPID Compared with Age-Matched Normal Horses

Glucocorticoids are known to exert inhibitory action on the hypothalamic-pituitary-thyroid axis. With recent evidence that free plasma cortisol and urinary excretion of cortisol metabolites may be increased in horses with pituitary pars intermedia dysfunction (PPID), it is important to further examine thyroid function in horses with PPID. To test the hypothesis that serum thyrotropin (TSH) concentrations are decreased in horses with PPID, baseline serum thyroid hormone and TSH concentrations, and responses to TSH-releasing hormone (TRH), were compared between 12 horses diagnosed as having PPID and 14 age-matched normal horses. Horses with PPID had resting serum concentrations of free thyroxine by equilibrium dialysis (fT4D) and TSH that were significantly lower than serum concentrations of fT4D and TSH in age-matched normal horses. Serum concentrations of total T4 and total and free triiodothyronine (T3) were also lower in horses with PPID compared with normal horses, but the differences did not reach statistical significance. Thyroid hormone and TSH responses to TRH administration were not different between horses with PPID and normal horses. In conclusion, serum fT4D concentrations are decreased in horses with PPID without an appropriate increase in serum TSH concentrations. Normal serum thyroid hormone and TSH concentration responses to exogenous TRH administration support the theory that increased glucocorticoid activity in horses with PPID may exert prolonged tonic suppression, but not complete inhibition, of TRH and subsequent TSH release, similar to what has been observed in other species.

TRH-induced secretion of adrenocorticotropin and cortisol in dogs with pituitary-dependent hypercortisolism

Background: In dogs, spontaneous Cushing’s syndrome is most often pituitary-dependent and caused by hypersecretion of adrenocorticotropic hormone (ACTH), resulting in increased adrenocortical glucocorticoid secretion similar to horses. In horses with Cushing’s syndrome (or pituitary pars intermedia dysfunction [PPID]) a thyrotropin-releasing hormone (TRH) stimulation test can be used for diagnosis, as TRH administration results in increased circulating ACTH and cortisol concentrations in affected horses.

Objective: The aim of this study was to investigate the effect of TRH administration on the circulating ACTH and cortisol concentrations in dogs with pituitary-dependent hypercortisolism (PDH).

Methods: Ten clinically normal control dogs and 10 dogs with PDH, all client owned, underwent a TRH stimulation test with measurement of plasma concentrations of ACTH and cortisol, before and after intravenous administration of 10 μg TRH/kg bodyweight.

Results: Plasma ACTH concentration did not rise significantly after TRH stimulation, neither in PDH dogs nor in clinically normal dogs. In contrast, the plasma cortisol concentration did increase significantly after TRH stimulation in both groups (p = .003 in PDH and p < .001 in control). Immunohistochemistry of normal adrenal glands demonstrated the presence of TRH receptors in the whole adrenal cortex.

Conclusions: The results of this study demonstrate that the TRH stimulation test should be rejected as a tool to diagnose PDH in dogs. The observed TRH-induced increase in plasma cortisol concentration without a significant rise in plasma ACTH concentration may be explained by a direct effect of TRH on adrenocortical cells mediated by adrenocortical TRH receptors.

Effect of dietary carbohydrates and time of year on ACTH and cortisol concentrations in adult and aged horses

Diagnosis of equine pituitary pars intermedia dysfunction (PPID) remains a challenge as multiple factors (stress, exercise, and time of year) influence ACTH and cortisol concentrations. To assess endocrine status in a study designed to evaluate the effects of age and diet on glucose and insulin dynamics, we performed thyrotropin-releasing hormone (TRH) stimulation tests and overnight dexamethasone suppression tests in March, May, August, and October on 16 healthy Thoroughbred and Standardbred mares and geldings. Horses were grouped by age: adult (mean ± SD; 8.8 ± 2.9 yr; n = 8) and aged (20.6 ± 2.1 yr; n = 8). None of the horses showed clinical signs (hypertrichosis, regional adiposity, skeletal muscle atrophy, lethargy) of pituitary pars intermedia dysfunction. Horses were randomly assigned to groups of 4, blocked for age, and fed grass hay plus 4 isocaloric concentrate diets (control, starch-rich, fiber-rich, and sugar-rich) using a balanced Latin square design. Data were analyzed using a multivariable linear mixed regression model. Baseline ACTH was significantly higher in aged horses (mean ± standard error of the mean; 60.0 ± 10.7 pg/mL) adapted to the starch-rich diet compared to adult horses (15.7 ± 12.0 pg/mL) on the same diet (P = 0.017). After controlling for age and diet, baseline ACTH concentrations were significantly increased in October (57.7 ± 7.1 pg/mL) compared to March (13.2 ± 7.1 pg/mL; P < 0.001), May (12.4 ± 7.1 pg/mL; P < 0.001), and August (24.2 ± 7.1 pg/mL; P < 0.001), whereas post-TRH ACTH was higher in August (376.6 ± 57.6 pg/mL) and October (370.9 ± 57.5 pg/mL) compared to March (101.9 ± 57.3 pg/mL; P < 0.001) and May (74.5 ± 57.1 pg/mL; P < 0.001). Aged horses had significantly higher post-dexamethasone cortisol on the starch-rich diet (0.6 ± 0.1 μg/dL) compared to the sugar-rich diet (0.2 ± 0.1 μg/dL; P = 0.021). Post-dexamethasone cortisol was significantly higher in October (0.6 ± 0.1 μg/dL) compared to March (0.3 ± 0.1 μg/dL; P = 0.005), May (0.2 ± 0.1 μg/dL; P < 0.001), and August (0.3 ± 0.1 μg/dL; P = 0.004). Breed did not influence ACTH or cortisol measurements. In conclusion, in addition to age and time of year, diet is a potential confounder as animals on a starch-rich diet may be incorrectly diagnosed with pituitary pars intermedia dysfunction.

Profiles of pro-opiomelanocortin and encoded peptides, and their processing enzymes in equine pituitary pars intermedia dysfunction

Equine pituitary pars intermedia dysfunction (PPID) is characterized by hyperplasia of the pars intermedia (PI) melanotrophs of the pituitary gland (PG), and increased production of proopiomelanocortin (POMC). POMC is cleaved by prohormone convertase 1 (PC1) to produce adrenocorticotropic hormone (ACTH), and further processing of ACTH by PC2 to produce alpha-melanocyte stimulating hormone (α-MSH) and corticotropin-like intermediate peptide (CLIP). High plasma ACTH concentrations in horses with PPID might be related to reduced conversion of ACTH to α-MSH by PCs. The hypothesis of this study was that PC1 and PC2 expression in the pituitary gland are altered in PPID, resulting in an abnormal relative abundance of POMC derived proteins. The objectives of this study were to identify the partial sequences of equine POMC, PC1, and PC2 mRNAs; and to determine whether the expression of POMC, PC1, and PC2 mRNAs in whole pituitary extracts, and POMC-protein in the cavernous sinus blood of horses are altered in PPID. We confirmed (RT-PCR and sequencing) that the partial sequences obtained match the corresponding regions of predicted equine POMC, PC1 and PC2 sequences. The expression (quantification by RT-qPCR) of POMC, PC1 and PC2 mRNAs were found upregulated in the pituitary of horses with PPID. Plasma (measured using RIA/ELISA) ACTH and α-MSH were elevated in PPID horses. These results indicate distinct differences in gene and protein expression of POMC and its intermediates, and processing enzymes in PPID. It provides evidence to support the notion that local, pituitary-specific inadequacies in prohormone processing likely contribute to equine PPID.

Metabolic and inflammatory responses to the common sweetener stevioside and a glycemic challenge in horses with equine metabolic syndrome

Extracts derived from the leaves of the stevia plant (stevioside) are commonly used as sweeteners for humans and horses. Stevioside appears to be safe for human consumption, including for individuals with insulin dysregulation. In the horse, the safety or metabolic effects of stevioside on normal animals or on those with metabolic dysfunction are unknown. Furthermore, the inflammatory response to a glycemic challenge or to stevioside in horses is not well defined. Therefore, the objective of this study was to measure the effects of stevioside and a glycemic challenge on insulin, glucose, and inflammatory responses in horses with a common metabolic dysfunction (equine metabolic syndrome or EMS) compared with non-EMS controls. To accomplish this, 15 horses were selected; 8 EMS and 7 age-matched controls. An oral sugar test was performed using Karo corn syrup (karo) or stevioside in a random crossover design. Horses were given 0.15 mL/kg body weight of karo or its equivalent grams of sugar in stevia dissolved in water. Blood samples were collected by jugular venipuncture before administration of either stevia or karo and at 60 and 240 min after administration. Serum was used for glucose and insulin determination and plasma for isolation of peripheral blood mononuclear cells (PBMCs) for inflammatory cytokine analysis via flow cytometry and reverse transcription PCR (RT-PCR). Stevia appeared to stimulate lower glycemic and insulinemic responses when compared to karo, in particular in EMS horses. EMS and control horses had inverse inflammatory responses to administration of either stevia or karo with EMS horses having a proinflammatory response (P ≤ 0.05). These data provide evidence as to why horses with EMS may be predisposed to developing laminitis, potentially as a result of an exaggerated inflammatory response to glycemic and insulinemic responses. Furthermore, the data provide new avenues for exploring mechanisms behind the syndrome, in particular when using a glycemic challenge.

Therapeutics for Equine Endocrine Disorders

Equine endocrine disease is commonly encountered by equine practitioners. Pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS) predominate. The most logical therapeutic approach in PPID uses dopamine agonists; pergolide mesylate is the most common. Bromocryptine and cabergoline are alternative drugs with similar actions. Drugs from other classes have a poor evidence basis, although cyproheptadine and trilostane might be considered. EMS requires management changes as the primary approach; reasonable justification for use of drugs such as levothyroxine and metformin may apply. Therapeutic options exist in rare cases of diabetes mellitus, diabetes insipidus, hyperthyroidism, and critical illness-related corticosteroid insufficiency.

Evaluation of a thyrotropin-releasing hormone solution stored at room temperature for pituitary pars intermedia dysfunction testing in horses

OBJECTIVE

To determine whether plasma ACTH concentrations vary following administration of a thyrotropin-releasing hormone (TRH) solution prepared for research purposes and stored at -20°C (rTRH) or prepared by a compounding pharmacy and stored at room temperature (approx 22°C; cTRH).

ANIMALS

34 adult horses.

PROCEDURES

The study consisted of 2 experiments. In experiment 1, each horse underwent 2 TRH stimulation tests separated by 24 hours; 10 horses were administered cTRH for the first test and rTRH for the second test (group 1), 10 horses were administered rTRH for the first test and cTRH for the second test (group 2), and 10 horses were administered rTRH for both tests (group 3). Plasma ACTH concentrations were measured at 0 (baseline) and 30 minutes after TRH administration and the delta ACTH responses (change in ACTH concentration after TRH administration) were calculated. In experiment 2, the design was the same as that for experiment 1 except there were 14 days between tests, ACTH was measured at 0 and 10 minutes after TRH administration, and 11, 9, and 10 horses were assigned to groups 1, 2, and 3, respectively.

RESULTS

Adverse effects associated with TRH administration included transient coughing and yawning. In experiment 1, the median delta ACTH response for the second test was significantly lower than that for the first test for all groups. In experiment 2, the median delta ACTH response did not differ significantly between the first and second tests for any group, ACTH concentrations after rTRH administration were positively correlated (rs = 0.95) with those after cTRH administration, and the mean ± SD bias in post-TRH ACTH concentration between rTRH and cTRH was 2.9 ± 12.4 pg/mL.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that the TRH stimulation test should not be repeated within 24 hours, and cTRH solution stored at room temperature could be used to effectively perform TRH stimulation testing in horses.

Equine pituitary pars intermedia dysfunction: current perspectives on diagnosis and management

Equine pituitary pars intermedia dysfunction (PPID) is a neurodegenerative disease of the hypothalamus, resulting in the loss of dopaminergic inhibition of pars intermedia. An oxidative stress injury of unknown etiology has been suggested to initiate the neurodegeneration. While hypertrichosis (formerly known as hirsutism) is considered pathognomic for advanced disease, the antemortem diagnosis of subclinical and early disease has continued to prove difficult. Numerous tests have been used with varying sensitivities and specificities. The overnight dexamethasone suppression test, originally documented to have 100% sensitivity and specificity in horses with advanced disease, has proven to be less valuable in identifying early disease. Basal plasma adrenocorticotropin concentrations have improved sensitivity and specificity when sampled during the autumn months, and α-melanocyte-stimulating hormone, while not yet commercially available, shows promise as a sensitive and specific single sample test. Recent advances in our knowledge include the strong association between laminitis and hyperinsulinemia, both common clinical signs associated with PPID. The pathogenesis of hyperinsulinemia, laminitis, and their association with this disease is a focus of current research. The dopamine agonist pergolide mesylate is still the mainstay of medical management, with studies on oral bioavailability, pharmacokinetics, and long-term survival rates now published.

Influence of feeding status, time of the day, and season on baseline adrenocorticotropic hormone and the response to thyrotropin releasing hormone-stimulation test in healthy horses

Equine pituitary pars intermedia function can be assessed by the measurement of baseline and thyrotropin releasing hormone (TRH)-induced concentrations of adrenocorticotropic hormone (ACTH); however, these measurements may be affected by the environment. Therefore, a prospective observational study evaluated the influence of feeding, time of the day, and season on baseline and TRH-induced concentrations of ACTH in healthy horses. Baseline ACTH was measured in 50 horses before and 2 h after feeding. Six research horses were subjected to a crossover study in which 6 TRH tests were performed in 2 different seasons, March-April (MA) and July-September (JS), at 2 different times of the day, 8 AM and 8 PM, and, under 2 different conditions relative to feeding status, fasted and 2 h after feeding. Differences between fasted and fed horses were found in baseline ACTH, 17.1 ± 1.8 versus 46.1 ± 7.6 pg/mL (P = 0.003) and TRH-stimulated ACTH: 124.1 ± 21.3 versus 192.6 ± 33.1 pg/mL (P = 0.029) at 10 min, and 40.1 ± 4.9 versus 73.2 ± 13.4 pg/mL (P = 0.018) at 30 min post TRH injection. No differences were found between tests performed at different times of the day. Basal ACTH concentrations were greater in JS than in MA, 17.1 ± 1.8 versus 11.9 ± 0.6 pg/mL (P = 0.006). A seasonal influence was also found in stimulated ACTH values, which were much greater in JS 122.7 ± 36.7 versus 31.2 ± 7.4 pg/mL, at 10 min (P = 0.03) and 39.0 ± 7.2 versus 19.8 ± 3.1 pg/mL, at 30 min (P = 0.03). In addition to season, feeding is a potential confounding factor when measuring baseline or stimulated ACTH in horses. In conclusion, feeding status should be standardized for the diagnosis of equine pituitary pars intermedia dysfunction.

Insulin dysregulation

Abnormalities of insulin metabolism include hyperinsulinaemia and insulin resistance, and these problems are collectively referred to as insulin dysregulation in this review. Insulin dysregulation is a key component of equine metabolic syndrome: a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses, ponies and donkeys. Insulin dysregulation can also accompany prematurity and systemic illness in foals. Causes of insulin resistance are discussed, including pathological conditions of obesity, systemic inflammation and pituitary pars intermedia dysfunction, as well as the physiological responses to stress and pregnancy. Most of the discussion of insulin dysregulation to date has focused on insulin resistance, but there is increasing interest in hyperinsulinaemia itself and insulin responses to feeding. An oral sugar test or in-feed oral glucose tolerance test can be performed to assess insulin responses to dietary carbohydrates, and these tests are now recommended for use in clinical practice. Incretin hormones are likely to play an important role in postprandial hyperinsulinaemia and are the subject of current research. Insulin resistance exacerbates hyperinsulinaemia, and insulin sensitivity can be measured by performing a combined glucose-insulin test or i.v. insulin tolerance test. In both of these tests, exogenous insulin is administered and the rate of glucose uptake into tissues measured. Diagnosis and management of hyperinsulinaemia is recommended to reduce the risk of laminitis. The term insulin dysregulation is introduced here to refer collectively to excessive insulin responses to sugars, fasting hyperinsulinaemia and insulin resistance, which are all components of equine metabolic syndrome.

Changes in plasma melanocyte-stimulating hormone, ACTH, prolactin, GH, LH, FSH, and thyroid-stimulating hormone in response to injection of sulpiride, thyrotropin-releasing hormone, or vehicle in insulin-sensitive and -insensitive mares

Six insulin-sensitive and 6 insulin-insensitive mares were used in a replicated 3 by 3 Latin square design to determine the pituitary hormonal responses (compared with vehicle) to sulpiride and thyrotropin-releasing hormone (TRH), 2 compounds commonly used to diagnose pituitary pars intermedia dysfunction (PPID) in horses. Mares were classified as insulin sensitive or insensitive by their previous glucose responses to direct injection of human recombinant insulin. Treatment days were February 25, 2012, and March 10 and 24, 2012. Treatments were sulpiride (racemic mixture, 0.01 mg/kg BW), TRH (0.002 mg/kg BW), and vehicle (saline, 0.01 mL/kg BW) administered intravenously. Blood samples were collected via jugular catheters at -10, 0, 5, 10, 20, 30, 45, 60, 90, and 120 min relative to treatment injection. Plasma ACTH concentrations were variable and were not affected by treatment or insulin sensitivity category. Plasma melanocyte-stimulating hormone (MSH) concentrations responded (P < 0.01) to both sulpiride and TRH injection and were greater (P < 0.05) in insulin-insensitive mares than in sensitive mares. Plasma prolactin concentrations responded (P < 0.01) to both sulpiride and TRH injection, and the response was greater (P < 0.05) for sulpiride; no effect of insulin sensitivity was observed. Plasma thyroid-stimulating hormone (TSH) concentrations responded (P < 0.01) to TRH injection only and were higher (P < 0.05) in insulin-sensitive mares in almost all time periods. Plasma LH and FSH concentrations varied with time (P < 0.05), particularly in the first week of the experiment, but were not affected by treatment or insulin sensitivity category. Plasma GH concentrations were affected (P < 0.05) only by day of treatment. The greater MSH responses to sulpiride and TRH in insulin-insensitive mares were similar to, but not as exaggerated as, those observed by others for PPID horses. In addition, the reduced TSH concentrations in insulin-insensitive mares are consistent with our previous observation of elevated plasma triiodothyronine concentrations in hyperleptinemic horses (later shown to be insulin insensitive as well).

Seasonal variation in results of diagnostic tests for pituitary pars intermedia dysfunction in older, clinically normal geldings

OBJECTIVE

To determine whether seasonal variations exist in endogenous plasma ACTH, plasma α-melanocyte-stimulating hormone (α-MSH), serum cortisol, and serum insulin concentrations and in the results of a dexamethasone suppression test for older, clinically normal geldings in Alabama.

DESIGN

Cohort study.

ANIMALS

15 healthy mixed-breed geldings (median age, 14 years).

PROCEDURES

Sample collection was repeated monthly for 12 months. Dexamethasone (0.04 mg/kg [0.02 mg/lb], IM) was administered and cortisol concentrations were determined at 15 and 19 hours. Radioimmunoassays were used to measure ACTH, α-MSH, cortisol, and insulin concentrations at each testing time. Hormone concentrations were compared between months via repeated-measures ANOVA and correlated with age within each month.

RESULTS

A significant time effect was found between months for α-MSH and insulin concentrations. Endogenous cortisol and ACTH concentrations remained within existing reference ranges. Significant correlations were detected between age and ACTH concentration for several fall and winter months and between age and insulin concentration for September.

CONCLUSIONS AND CLINICAL RELEVANCE

Older horses have higher ACTH concentrations in several fall and winter months and higher insulin concentrations in September than do younger horses. Seasonally specific reference ranges are required for α-MSH and insulin concentrations, with significantly higher concentrations detected in the fall. Practitioners should be advised to submit samples only to local laboratories that can provide such reference ranges for their local geographic region.

Circadian and circannual rhythms of cortisol, ACTH, and α-melanocyte-stimulating hormone in healthy horses

Cosinor analysis was used to evaluate whether pituitary and adrenal hormones exhibit circadian rhythmicity in horses. The effect of season and animal age on their respective rhythms was also determined. In addition, the usefulness of evaluating cortisol rhythmicity for the diagnosis of pituitary pars intermedia dysfunction (PPID) was assessed. Serum cortisol concentrations (P < 0.01), but not plasma ACTH or α-melanocyte-stimulating hormone (α-MSH), showed a significant circadian periodicity in horses. An effect of season on hormone concentration was observed with plasma ACTH and α-MSH concentration greater in the fall and cortisol concentration greater in the spring (P < 0.001). Age did not affect cortisol rhythm, but it did blunt the variation in cortisol concentration in horses, similar to what has been previously reported to occur in aged people and dogs. In addition, our results suggest that clinically and diagnostically normal, non-PPID-affected horses commonly have a loss of cortisol diurnal rhythm. Therefore, measurement of circadian rhythm is not an appropriate diagnostic test for PPID.

Evaluation of basal plasma α-melanocyte-stimulating hormone and adrenocorticotrophic hormone concentrations for the diagnosis of pituitary pars intermedia dysfunction from a population of aged horses

REASONS FOR PERFORMING STUDY

The sensitivity and specificity of basal plasma α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotrophic hormone (ACTH) for the diagnosis of pituitary pars intermedia dysfunction (PPID) has not been evaluated in a population-based study.

OBJECTIVES

To evaluate basal plasma α-MSH and ACTH concentrations for the diagnosis of PPID in a population of horses aged ≥ 15 years.

METHODS

Owner-reported data were obtained using a postal questionnaire distributed to an equestrian group. A subgroup of surveyed owners was visited and veterinary examination performed on horses aged ≥ 15 years. Blood samples were analysed for plasma α-MSH and ACTH concentrations. Seasonally adjusted cut-off values for α-MSH and ACTH concentrations for the diagnosis of PPID were obtained using Youden index values against a clinical gold standard diagnosis (hirsutism plus 3 or more clinical signs of PPID).

RESULTS

α-melanocyte-stimulating hormone and ACTH were highly correlated with each other and with clinical and historical indicators of PPID. The increase in both α-MSH and ACTH with increasing numbers of clinical signs in affected horses supports a spectrum of disease. Both variables were affected by season, with derived cut-off values being higher in autumn compared with other seasons. Sensitivity and specificity were moderate and good in nonautumn seasons (59 and 93%, respectively) for α-MSH using a cut-off of 52.0 pmol/l. Sensitivity and specificity were good in nonautumn seasons (80 and 83%, respectively) for ACTH using a cut-off of 29.7 pg/ml. For both α-MSH and ACTH, sensitivity and specificity were close to 100% for samples obtained during the autumn period.

CONCLUSIONS AND POTENTIAL RELEVANCE

Basal plasma α-MSH and ACTH had moderate-to-good sensitivity and specificity for the diagnosis of PPID, which improved substantially during the autumn period, suggesting this may be the ideal time to test. Further studies to develop seasonally adjusted reference intervals for different geographical locations are warranted.

Equine pituitary pars intermedia dysfunction

Equine pituitary pars intermedia dysfunction (PPID), also known as equine Cushing's syndrome, is a widely recognized disease of aged horses. Over the past two decades, the aged horse population has expanded significantly and in addition, client awareness of PPID has increased. As a result, there has been an increase in both diagnostic testing and treatment of the disease. This review focuses on the pathophysiology and clinical syndrome, as well as advances in diagnostic testing and treatment of PPID, with an emphasis on those findings that are new since the excellent comprehensive review by Schott in 2002.

Evaluation of plasma ACTH, alpha-melanocyte-stimulating hormone, and insulin concentrations during various photoperiods in clinically normal horses and ponies and those with pituitary pars intermedia dysfunction

OBJECTIVE

To measure plasma ACTH, alpha-melanocyte-stimulating hormone (aloha-MSH), and insulin concentrations during various photoperiods between February and October in horses and ponies with and without pituitary pars intermedia dysfunction (PPID).

DESIGN

Cohort study.

ANIMALS

13 clinically normal (control) ponies, 14 clinically normal (control) horses, 7 ponies with PPID, and 8 horses with PPID.

PROCEDURES

Blood samples were collected from February through October during 8 photoperiods: 1, February 13 through March 2; 2, April 4 through 6; 3, June 19 through 22; 4, August 6 through 7; 5, August 14 through 17; 6, September 4 through 6; 7, September 26 through 28; and 8, October 16 through 18. Plasma ACTH, alpha-MSH, and insulin concentrations at each photoperiod were compared among groups.

RESULTS

Log ACTH concentration was increased during photoperiod 4 through 8, compared with photoperiod 1 through 3, in all groups. In photoperiod 3 through 7, log ACTH concentrations were higher in horses and ponies with PPID, compared with values for control horses and ponies. alpha-Melanocyte-stimulating hormone (log and raw value) concentration was higher in photoperiod 2 through 8, compared with photoperiod 1, in control horses and ponies. In horses and ponies with PPID, log alpha-MSH concentration was higher in photoperiod 3 through 8, and alpha-MSH concentration was higher in photoperiod 4 through 8, compared with photoperiod 1. In control horses and ponies, plasma insulin concentration was lower in photoperiod 3 than in photoperiod 1.

CONCLUSIONS AND CLINICAL RELEVANCE

Plasma alpha-MSH and ACTH concentrations increased as daylight decreased from summer solstice (maximum daylight hours) to 12 hours of daylight.