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Topic:Endocrine System
The endocrine system in horses comprises a network of glands and hormones that regulate various physiological processes, including metabolism, growth, reproduction, and stress response. Key components of the equine endocrine system include the pituitary gland, thyroid gland, adrenal glands, and pancreas. Hormones such as insulin, cortisol, and thyroid hormones are produced and released into the bloodstream to maintain homeostasis and respond to internal and external stimuli. Dysregulation of the endocrine system can lead to conditions such as Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction) and Equine Metabolic Syndrome. This page compiles peer-reviewed research studies and scholarly articles that explore the structure, function, and disorders of the endocrine system in horses, providing insights into its impact on equine health and management.
Disorders of the equine thyroid gland. Regulatory control of the thyroid gland in horses is similar to other species. Clinical signs of hypothyroidism in adult horses are minimal. Several drugs and physiologic and pathophysiological states can cause circulating thyroid hormone concentrations to be low without actual pathology of the thyroid gland. Thus, nonthyroidal factors must be ruled out before a diagnosis of hypothyroidism can be made. Thyroid hormone supplementation seems to be well tolerated, even in euthyroid horses. Neonatal foals have very high circulating thyroid hormone concentrations, and deficiencies result in signif...
The endocrine disruptive effects of ergopeptine alkaloids on pregnant mares. During equine gestation, ergopeptine alkaloid exposure is not uncommon, and pregnant mares are particularly sensitive to the endocrine disruptive effects of these compounds on lactogenesis and steroidogenesis. Agalactia, prolonged gestation, abortion, dystocia, and placental and fetal abnormalities are all clinical manifestations of changes in the endocrine milieu induced by the ingestion of ergopeptine alkaloid-contaminated feedstuffs by mares during late gestation. An understanding of the endocrine disruptive effects of gestational exposure to ergopeptine alkaloids is necessary for the diagn...
Hypothalamic-pituitary gland axis function and dysfunction in horses. Hypothalamic-pituitary (HP) dysfunction has been documented in a limited capacity in horses and foals associated with critical illness, stress, and pain. This article reviews species-specific details of anatomy, function, hormones, receptors, and testing of the HP axis in the horse. A discussion of critical care medicine relevant to HP dysfunction in the horse with some reference to current understanding in human medicine is made, focusing primarily on current and relevant literature. A brief mention of other conditions described in human and veterinary medicine is also provided for reference ...
Reproductive cycles of horses. Horses are long-day breeders. During the breeding season, cycle length is about 22 days with 5-7 days of oestrus. Gonadotroph cells are localized in the pars distalis as well the pars tuberalis of the pituitary and heterogeneity in the pattern of LH and FSH storage within the gonadotroph population is considered the basis for the differential regulation of gonadotrophin secretion throughout the reproductive cycle. No short and distinct periovulatory LH peak exists in the mare. The equine ovary has an extreme large size and weight. One to two major follicular waves develop per cycle. The preovu...
Expression of steroidogenic enzymes during equine testicular development. In the mammalian testis, Leydig cells are primarily responsible for steroidogenesis. In adult stallions, the major endocrine products of Leydig cells include testosterone and estrogens. 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4)-isomerase (3βHSD) and 17α-hydroxylase/17,20-lyase (P450c17) are two key steroidogenic enzymes that regulate testosterone synthesis. Androgens produced by P450c17 serve as substrate for estrogen synthesis. The aim of this study was to investigate localization of the steroidogenic enzymes P450c17, 3βHSD, and P450arom and to determine changes in expression during dev...
Supplementation of equine early spring transitional follicles with luteinizing hormone stimulates follicle growth but does not restore steroidogenic activity. This study was conducted to test the hypothesis that supplementation of growing follicles with LH during the early spring transitional period would promote the development of steroidogenically active, dominant follicles with the ability to respond to an ovulatory dose of hCG. Mares during early transition were randomly assigned to receive a subovulatory dose of equine LH (in the form of a purified equine pituitary fraction) or saline (transitional control; n = 7 mares per group) following ablation of all follicles >15 mm. Treatments were administered intravenously every 12 h from the day th...
Effect of a povidone-iodine intrauterine infusion on progesterone levels and endometrial steroid receptor expression in mares. Intrauterine infusions have been widely used for the treatment of endometritis in the mare. Nevertheless, their consequences on endocrine and endometrial molecular aspects are unknown. We studied the effect of a 1% povidone-iodine solution intrauterine infusion on progesterone levels, endometrial histology and estrogen (ERα) and progesterone (PR) receptor distribution by immunohistochemistry. Methods: Fourteen healthy mares were used in this study. Estruses were synchronized and seven mares were treated with intrauterine infusions at days 0 and 2 post ovulation of two consecutive estrous cycl...
Insulin, glucagon, and leptin in critically ill foals. Endocrine dysregulation of hormones of energy metabolism is well documented in critically ill humans, but limited information exists in septic foals. The purpose of this study was to provide information on the hormonal response to energy metabolism in critically ill foals, focusing on insulin, glucagon, and leptin. Objective: Concentrations of insulin, glucagon, leptin, and triglycerides will be higher, whereas glucose concentration will be lower in septic foals than in healthy and sick nonseptic foals. The magnitude of these differences will be associated with severity of disease and nonsurvi...
Insulin-like growth factor-I (IGF-I) protects cultured equine Leydig cells from undergoing apoptosis. Leydig cells located in the interstitial space of the testicular parenchyma produce testosterone which plays a critical role in the maintenance and restoration of spermatogenesis in many species, including horses. For normal spermatogenesis, maintaining Leydig cells is critical to provide an optimal and constant level of testosterone. Recently, an anti-apoptotic effect of IGF-I in testicular cells in rats has been reported, but a similar effect of IGF-I on equine Leydig cells remains to be elucidated. If IGF-I also protects stallion testicular cells from undergoing apoptosis, then IGF-I may ha...
Comparison between blood serum and salivary cortisol concentrations in horses using an adrenocorticotropic hormone challenge. In horses, serum cortisol concentration is considered to provide an indirect measurement of stress. However, it includes both free and bound fractions. The sampling method is also invasive and often stressful. This is not the case for salivary cortisol, which is collected using a more welfare-friendly method and represents a part of the free cortisol fraction, which is the biologically active form. Objective: To compare salivary and serum cortisol assays in horses, in a wide range of concentrations, using an adrenocorticotropic hormone (ACTH) stimulation test, in order to validate salivary cor...
Positive effect of FSH but not LH on early development of the dominant follicle in mares. The effects of FSH, LH or both on follicular growth and intrafollicular free insulin-like growth factor (IGF)-1 and oestradiol were investigated in mares after the beginning of deviation (largest follicle >/= 20 mm; Hour 0). A single treatment with a gonadotropin-releasing hormone antagonist (acyline) was given at Hour 3 to suppress the concentrations of FSH and LH. Five groups (n = 5 mares per group) were evaluated in the present study: (1) control; (2) acyline treated; (3) acyline + recombinant equine (re) FSH treated; (4) acyline + reLH treated; and (5) combined acyline + reFSH + reLH tr...
Field treatment and management of endocrinopathic laminitis in horses and ponies. Endocrinopathic laminitis in horses and ponies has the potential to cause extensive damage to the feet before clinical signs of laminitis are apparent. Early diagnosis and intervention based on the detection and control of hyperinsulinemia are critical if the crippling changes that are seen in the disease are to be avoided. Preventing endocrinopathic laminitis, rather than treating it, is good practice management benefiting not only equine patients but also their owners.
Association of season and pasture grazing with blood hormone and metabolite concentrations in horses with presumed pituitary pars intermedia dysfunction. Pituitary pars intermedia dysfunction (PPID) is a risk factor for pasture-associated laminitis, which follows a seasonal pattern. Objective: Hormonal responses to season differ between PPID and unaffected horses. Methods: Seventeen horses aged 8-30 years (14 horses ≥ 20 years of age). Methods: Longitudinal observational study. Blood was collected monthly from August 2007 until July 2008 after pasture grazing and again after overnight stall confinement. Blood hormone and metabolite concentrations were measured and pasture grass samples were analyzed to determine carbohydrate content. Analysis...
Cortisol release, heart rate, and heart rate variability in transport-naive horses during repeated road transport. Domestic animals are often repeatedly exposed to the same anthropogenic stressors. Based on cortisol secretion and heart rate, it has been demonstrated that transport is stressful for horses, but so far, changes in this stress response with repeated road transport have not been reported. We determined salivary cortisol concentrations, fecal cortisol metabolites, cardiac beat-to-beat (RR) interval, and heart rate variability (HRV) in transport-naive horses (N = 8) transported 4 times over a standardized course of 200 km. Immunoreactive salivary cortisol concentrations always increased in respon...
Blood steroid concentrations in domestic Mongolian horses. Traditionally, analysis of blood cortisol alone has been used to evaluate adrenal function. Currently, multisteroid analyses are considered more informative than analysis of a single hormone to assess adrenal function. The objective of the present research was to create a database for steroid reference values for domestic Mongolian horses. Seven adrenal steroid levels were determined in the blood of 18 colts, 34 stallions, 25 geldings, 17 fillies, and 29 mares. Results were as follows (lowest and highest group median, in nanograms per milliliter): progesterone: <0.030 (fillies), 4.30 (mares), ...
Serum thyroid hormone, insulin, glucose, triglycerides and protein concentrations in normal horses: association with topical dexamethasone usage. The aim of this study was to determine if topical application of dexamethasone affected the serum concentrations of thyroid hormones (triiodothyronine T(3) and thyroxine T(4)), glucose, triglycerides, total protein and insulin in normal horses. Ten horses were treated twice daily for 10 days with 50 g dexamethasone using an ointment formulation. Thyroid hormones and insulin were assayed using standard radioimmunoassay methods, while glucose, triglycerides and total protein were determined using a standard enzymatic method and the Biuret reaction, respectively. An increase in serum glucose and ...
Differences in seasonal changes of fecal androgen levels between stabled and free-ranging Polish Konik stallions. Blood and feces samples were collected from Polish Konik stallions kept under conventional stable conditions and in the forest reserve during a 1-year study period. Levels of testosterone (T) and androstenedione (A(4)) were measured using radioimmunoassay. Positive correlation between fecal and plasma concentrations of androgens was observed. Fecal T concentrations increased in April and May reaching peak value mid-April in the stallions from the reserve group and 2 weeks later in the stallions from the stable group. Comparatively, concentrations of T were higher in the stable group. Levels of...
The use of adrenocorticotrophic hormone as a potential biomarker of pituitary pars intermedia dysfunction in horses. Elevated concentrations of plasma adrenocorticotrophic hormone (ACTH) in horses with pituitary disturbances are often associated with dysfunction of the pituitary's pars intermedia. The majority of such animals exhibit an increased susceptibility to laminitis, particularly during the autumn. The 24h plasma ACTH profiles of horses with pituitary pars intermedia dysfunction (PPID), and those of matched controls, were determined in March, June, September and December. Differences in ACTH concentrations between the groups were significant (P<0.0001), regardless of photoperiod, and persisted thr...
Endocrinopathic laminitis. In laminitis occurring in the field, as opposed to laminitis occurring during hospitalization or severe illness, endocrinopathic laminitis is the predominant form of laminitis. Prevalent causes of endocrinopathic laminitis are ECS and EMS. Exclusion of inflammatory or weight bearing causes of laminitis and focussing on the identification and treatment of underlying endocrine conditions will improve laminitis management strategies.
Endocrinopathic laminitis: reducing the risk through diet and exercise. Certain individual animals appear predisposed to recurrent pasture-associated laminitis, but the exact mechanisms underlying their predisposition remain a fundamental question in laminitis research. The risk of endocrinopathic laminitis can be reduced if steps are taken to improve insulin sensitivity and reduce inflammation using strategies based on exercise and diet. Exercise has been shown to reduce insulin resistance and suppress inflammation, and dietary manipulation can significantly affect insulin sensitivity.
The pharmacologic basis for the treatment of endocrinopathic laminitis. Although the treatment and management of laminitis in the horse requires a holistic and often multidisciplinary approach from the veterinarian, farrier, and nutritionist, this review focuses on pharmacologic interventions that might have prophylactic benefit, specifically in the horse with laminitis as a result of pituitary pars intermedia dysfunction and equine metabolic syndrome.
Corticosteroid-associated laminitis. A direct causal association between corticosteroid use and laminitis has yet to be proven scientifically, and there have been few studies specifically addressing this aspect. New evidence, however, is improving the understanding of the causes of laminitis, particularly related to endocrine factors. The focus of this article is discussing the circumstances under which steroids might cause this condition.
Endocrine alterations around the time of abortion in mares impregnated with donkey or horse semen. The objective of this study was to monitor and compare the concentrations of equine chorionic gonadotropin (eCG), progesterone and estrone sulphate during normal and failed pregnancies of mares impregnated with donkey or horse semen, relating their individual endocrine profiles to the time of pregnancy loss, and to the histopathologic findings in the aborted fetuses and placenta. Mares (n=54) were used, 32 of them impregnated with donkey semen and 22 impregnated with horse semen. Blood samples were taken twice a week from Day 35 to 120 of pregnancy. Ultrasonographic observations of the fetus w...
Hyperleptinemia in mares and geldings: assessment of insulin sensitivity from glucose responses to insulin injection. Four experiments were conducted 1) to assess the use of glucose responses to insulin injections as a means of estimating insulin sensitivity in horses and 2) to compare the insulin sensitivities of normal horses vs. those displaying hyperleptinemia (HL). In Exp. 1, HL mares and geldings (n = 4 each) and 4 mares and geldings with normal leptin concentrations (NL) were injected intravenously with 20 and 100 mU/kg of BW of bovine insulin on 2 separate occasions in December 2008. In Exp. 2, the experimental protocol was repeated in late April 2009. In Exp. 1, the 20 mU/kg of BW dose of insulin cau...
Seasonal variation in serum concentrations of selected metabolic hormones in horses. Determination of adrenocorticotropic hormone (ACTH) concentration is a commonly used test in the evaluation of endocrine causes of equine laminitis, but the concentration in healthy horses can be high at certain times of year, which alters the specificity of the ACTH test. Objective: To determine if circulating concentrations of ACTH, cortisol, glucose, insulin, and thyroxine vary month to month in healthy horses and in horses with equine metabolic syndrome (EMS). Methods: Nine healthy adult horses were studied on their farm/stable over the course of 1 year. After the diagnosis of EMS, 10 lami...
Role of prolactin in the gonadotroph responsiveness to gonadotrophin-releasing hormone during the equine annual reproductive cycle. A combined suppressive effect of prolactin (PRL) and dopamine on the secretion of luteinising hormone (LH) at the level of the pituitary gland has been identified in sheep, a short-day breeder. However, little is known about the role of PRL in the intra-pituitary regulation of the gonadotrophic axis in long-day breeders. In the present study, we investigated the effects of PRL on LH and follicle-stimulating hormone (FSH) secretion during the equine annual reproductive cycle. Horse pituitaries were obtained during the breeding season (BS) and nonbreeding season (NBS). Cells were dispersed, plat...
Measurement of C-peptide concentrations and responses to somatostatin, glucose infusion, and insulin resistance in horses. Hyperinsulinaemia is detected in horses with insulin resistance (IR) and has previously been attributed to increased pancreatic insulin secretion. Connecting peptide (C-peptide) can be measured to assess pancreatic function because it is secreted in equimolar amounts with insulin and does not undergo hepatic clearance. Objective: A human double antibody radioimmunoassay (RIA) detects C-peptide in equine serum and concentrations would reflect responses to different stimuli and conditions. Methods: A validation procedure was performed to assess the RIA. Six mature mares were selected and somatos...
Species specific thyroid signal transduction: conserved physiology, divergent mechanisms. In the literature, data obtained in signal transduction from various species thyroids and cells lines are often integrated in a common model. We investigate qualitatively and systematically, using the same protocol, the control by TSH of the two main functions of the thyrocytes, the synthesis and the secretion of thyroid hormones. In all species investigated, the TSH receptor activates both. In some species, including humans, rats and mice, the TSH receptor activates both the cAMP and phospholipase C-PIP2 cascades, in others (e.g. dog) it only stimulates the first. The cAMP pathway activates t...
