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Topic:Hormones
Hormones in horses are chemical messengers produced by various glands and tissues, regulating numerous physiological processes essential for maintaining homeostasis. These hormones influence a wide range of functions, including growth, metabolism, reproduction, and stress responses. Key hormones in equine physiology include cortisol, estrogen, testosterone, and insulin, among others. The levels and effects of these hormones can vary based on factors such as age, sex, and environmental conditions, impacting overall health and performance. This page compiles peer-reviewed research studies and scholarly articles that explore the production, regulation, and physiological roles of hormones in equine biology.
Evaluation of a new immunoassay for measuring equine serum insulin and plasma adrenocorticotrophic hormone: A method comparison study. Insulin dysregulation (ID) and pituitary pars intermedia dysfunction (PPID) are common equine endocrinopathies detected by measurement of circulating insulin and adrenocorticotrophic hormone (ACTH) concentrations. Different assays may yield variable results, limiting comparability. Objective: To compare immunoassays developed specifically to measure equine insulin (VIDAS® Equine Insulin; enzyme-linked fluorescent assay [ELFA]) and equine ACTH (VIDAS® Equine ACTH; ELFA) with results obtained from a chemiluminescence immunoassay (CLIA) measured using IMMULITE 2000XPi, a chemiluminescence enzym...
Randomized study of the effects of testosterone administration on severity of asthma in horses. Low testosterone concentrations are associated with poor lung function in human asthmatics, and androgen administration improves airway obstruction in women with asthma. Objective: Testosterone improves clinical scores, lung function, airway neutrophilia, and airway remodeling in horses with severe asthma. Methods: Ten horses affected by severe asthma from a research herd. Methods: In a randomized, blinded, crossover study, horses received dexamethasone (0.06 mg/kg PO q24h for 10 days) or a single intramuscular injection of testosterone cypionate (0.35 mg/kg), with a 2-week washout period. ...
Training- and competition-related changes on circulating adrenocorticotropic hormone and cortisol response in Thoroughbred Racehorses. Exercise is a physiological challenge inducing intensity-dependent hypothalamic-pituitary-adrenal (HPA) axis responses. The hypothesis was that training and racing sessions induce different HPA axis activation. Objective: To assess whether different exercise sessions affect equine adrenal responsiveness according to age and sex. Methods: Ten clinically healthy Thoroughbred racehorses (6 females and 4 males) were recruited. Four were 2 years old and six were 3 years old. Horses were subjected to two exercise conditions: training and competitive racing sessions. Blood samples were collected at r...
Pituitary Pars Intermedia Dysfunction. Pituitary pars intermedia dysfunction (PPID) is one of the most common equine endocrine disorders. It is caused by oxidative damage to dopaminergic neurons in the hypothalamus and results in loss of inhibition of the pituitary pars intermedia. This leads to increased circulating concentrations of adrenocorticotropic hormone (ACTH) and related peptides. Diagnosis is based on increased resting and/or thyrotropin releasing hormone-stimulated ACTH concentrations. Assessment of insulin regulation is also important, as insulin dysregulation occurs in at least one-third of PPID cases and is associate...
Endocrine and uterine hemodynamic differences between intra- and interspecific equine pregnancies during early gestation. Interspecific equine pregnancies (mare x donkey) are associated with altered endocrine patterns and increased pregnancy loss; however, the relationship between hormonal dynamics, luteal activity, and uterine hemodynamics in relation to conceptus origin remains incompletely understood. This study aimed to evaluate the influence of conceptus origin on endocrine profiles, supplementary corpora lutea (SCL) formation, and uterine hemodynamics, and to investigate the relationships among these variables. Twenty mares were initially included; however, six were excluded due to pregnancy loss or incompl...
Hormonal, clinical, and survival outcomes in horses and ponies diagnosed with pituitary pars intermedia dysfunction in primary care practice. Variables associated with hormonal, clinical, and survival outcomes of equids with pituitary pars intermedia dysfunction (PPID) are incompletely investigated. Objective: Evaluate the effect of initial hormone concentrations, clinical signs, treatment, and owner-related variables on outcomes in horses and ponies with PPID. Methods: Medical records from 213 horses and ponies diagnosed with PPID by 2 university-based primary care practices. Methods: Analytical observational cohort study. Variables associated with adrenocorticotropic hormone normalization, clinical improvement, and survival after ...
The effects of clomiphene citrate on reproductive parameters in the mare. Clomiphene citrate (CC) is a longstanding human fertility drug that has yet to be investigated in horses. The objective was to evaluate effects of CC on mare reproduction. After follicle ablation on Day 0, treatment mares (n=5) received 2 grams of CC intravenously on Days 1-5 while control mares (n=4) received no treatment. Transrectal ultrasonography was performed on Days 1-5 and 8, and transvaginal oocyte aspiration occurred on Day 8. Hormones were analyzed on Days 0-5, 8, 11, and 15. Mares receiving CC exhibited higher uterine edema grades compared to control mares (P=0.009). Antral follicl...
Hypothalamic-pituitary-adrenal axis response and glucose homeostasis during exercise to fatigue before and after training in old and young adult Standardbred mares. While reduced cortisol responses to exercise have been described in older horses, the effects of aging and training on the hypothalamic-pituitary-adrenal axis (HPA axis) and downstream metabolic signaling are currently unknown. Unassigned: This study tested the hypothesis that age and exercise training alter the response of the HPA axis, insulin, and glucose during an acute exercise challenge to fatigue. Methods: Six old (22.0±0.7 years) and six young adult (7.3±0.6 years) unfit Standardbred mares were tested. Mares ran a graded exercise test before (GXT1) and after (GXT2) 8 weeks of trainin...
Compartmentalization of DHCR7 links 7-DHC metabolism to B-ring unsaturated steroid synthesis in the equine fetal gonad. The endocrinology of equine pregnancy is unique, but many of its unusual phenomena closely resemble pregnancy in women. Placental estrone synthesis uses fetal adrenal androgen precursors in women, but from the fetal gonads in mares, peaking midway through the 11-month gestation. Unique to equine pregnancies, other estrogens like equilin, typified by an unsaturated B-ring sterol structure, are synthesized from 7-dehydrocholesterol (7-DHC), the immediate precursor to cholesterol, without cholesterol formation, peaking later in gestation. Currently, the spatial and temporal regulation underlying ...
Development and validation of a liquid chromatography-tandem mass spectrometry method for the quantification of twenty-five steroids in equine serum. Steroids are potential biomarkers for monitoring equine pregnancy. However, immunoassays currently used for their quantification suffer from cross-reactivity and limited specificity, thus requiring more accurate methods. This study reports the development and validation of a robust liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for simultaneous quantification of 25 steroids covering the main biosynthetic pathways of progestogens, corticosteroids, androgens, and estrogens. Steroids were extracted by protein precipitation followed by evaporation, derivatization, and reconstitut...
Vasopressin stimulation testing in hospitalized foals. Hypothalamic-pituitary-adrenal gland axis (HPAA) dysfunction impairs survival and can be diagnosed with an arginine vasopressin (AVP) stimulation test. Objective: Adrenocorticotropic hormone (ACTH) and cortisol response to an AVP stimulation test will be attenuated in hospitalized foals compared to healthy foals and associated with severity of disease. Methods: Fifteen healthy and 45 hospitalized foals: 19 septic, 26 sick non-septic (SNS). Methods: A prospective, observational case-control design was utilized to assess HPAA function and compare corticotropin-releasing hormone, ACTH, and cortis...
Assessment of Thyroid Hormones Using the Immulite 2000xpi Analyzer in Healthy Donkeys. The Immulite 2000xpi, a chemiluminescent immunoassay (CLIA)-based analyzer, is currently recommended for measuring thyroid hormones (TH) concentrations in equids. However, scarce data are available on TH measurements in donkeys using this technique. Similarly, the effects of factors such as age and gender on TH concentrations have not been assessed with this analyzer. Serum free and total T4 and T3 concentrations were measured in 40 healthy donkeys (6.1 ± 0.8 years old; 8 intact jacks and 32 jennies) and 41 horses (10.7 ± 0.5 years old; 15 stallions and 26 geldings) using the Immulite 2000xp...
Preliminary investigation of ghrelin in horses and ponies: receptor expression and associations with prandial state, morphometry and signalment. The hormone ghrelin has had limited attention in Equidae, despite connections with appetite and metabolic health in other species. Ghrelin influences hunger and food intake, energy expenditure and storage, glucose metabolism, and thermoregulation, through its receptor-mediated effects on tissues and via interaction with other hormones. There are two circulating forms, acylated and des-acylated ghrelin, considered to have distinct effects on metabolism in other species. The aims of this study were to confirm the presence of ghrelin receptors in equine tissues, to validate an assay for the measu...
Hair steroid hormones concentrations in jennies and their donkey foals from parturition to 6 months postpartum. The period after parturition/birth represents a delicate time for the dam and the offspring, during which several physiologic changes occur under finely tuned hormonal control, with the hypothalamic-pituitary- adrenal (HPA) axis playing a key role. This period, well studied in horses, received little attention in donkeys, despite the importance of species-specific knowledge for population preservation programs in many endangered donkey breeds. The present study aimed to investigate cortisol (C), DHEA(S), 17-β-estradiol (E2), progesterone (P4), testosterone (T), and allopregnanolone (AlloP) ha...
Follicular growth and atresia at the time of oocyte aspiration differentially affect in vitro embryo production in mares. Ovum pick-up (OPU) followed by intracytoplasmic sperm injection (ICSI) is increasingly used in horses, yet embryo production rates remain variable, potentially reflecting differences in oocyte developmental competence at the time of aspiration. We investigated whether the stage of the follicular wave, growth phase (GP) versus atresia phase (AP), and associated endocrine profiles in mares, influence oocyte developmental competence. Mares underwent follicular ablation to synchronize follicular waves, and oocytes were aspirated during GP or AP for in vitro production of embryos. Serum concentrati...
Maternal age-related changes in renin-angiotensin-aldosterone system and electrolyte homeostasis during equine pregnancy. The renin-angiotensin-aldosterone system (RAAS) plays a key role in fluid and electrolyte homeostasis during pregnancy. In humans and laboratory animals, maternal age has been shown to modulate RAAS activity, potentially altering the secretion of renin (REN), angiotensin II (ANG-II), and aldosterone (ALD), thereby influencing sodium (Na⁺) and potassium (K⁺) regulation. Objective: To evaluate whether maternal age influences RAAS activity in pregnant mares. Methods: Forty-five Spanish Purebred mares were monitored monthly over 11 months of pregnancy and categorized into three age groups: You...
Evidence of Physiological Comodulation During Human-Animal Interaction: A Systematic Review. This review examines the evidence in the literature for physiological co-modulation during human-animal interaction, identifying studies that assessed comodulation via simultaneous measurement of physiological signals in both species, performing quantitative comparisons. We searched (last search: August 5, 2025) PubMed, EMBASE, Scopus, Google Scholar, Animal Studies Repository, Cochrane, and the Consensus App academic search engine. Risk of bias was assessed using an adapted version of the ROBINS-I V2 tool. The results, grouped by data analysis method, interaction context, and physiological pa...
Physiological determinants of plasma biochemical parameters in horses: a structured review. The biochemical profile of horses is a cornerstone in equine clinical diagnostics, offering valuable insights into health status, disease progression, and physiological adaptation to internal and external stimuli. However, the interpretation of these biochemical parameters is far from straightforward, as they are subject to significant variation influenced by a range of physiological factors. Age, sex, breed, level of physical activity, nutritional condition, and reproductive status can all modulate baseline biochemical values, potentially leading to diagnostic inaccuracies if not properly con...
Annual Variation in Stallion Testicular Measurements and Its Association With Hormonal Profiles and Semen Quality. In a stallion breeding-soundness examination, testicular volume may be estimated either from total scrotal width or by measuring the height, width and length of both testes. This study aimed to determine whether testicular morphometric parameters obtained by ultrasonography fluctuate throughout the year, and whether these variations affect semen quality and hormone production in stallions located at low latitudes in the Southern Hemisphere. Eight healthy light-breed stallions were evaluated every 15 days for 1 year, encompassing the breeding (October-April) and non-breeding(May-September) ...
Ovarian follicle dynamics in the plains zebra (Equus quagga). Understanding follicular dynamics during the estrous cycle is essential for developing breeding programs and assisted reproductive technologies (ARTs) in any species. In this context, knowledge of follicular dynamics and related uterine changes in the plains zebra (Equus quagga) is essential to increase the declining population of this vital African species and to provide comparative insights into more endangered zebra species. Likely due to the difficulty of handling these wild species, no detailed reproductive study using ultrasonography has been performed in any zebra. Therefore, the presen...
Effects of Dietary Zinc Cysteamine Supplementation on Growth Performance, Physiological Responses, and Fecal Microbiota in Weaned Foals. Zn-CS chelate has shown beneficial effects on gut health and growth in piglets, but its impact on weaned foals remains largely unknown. This study evaluated the effects of dietary Zn-CS supplementation on growth performance, nutrient digestibility, physiological status, and fecal microbiota in weaned Ili foals. Thirty-two six-month-old foals were randomly assigned to four treatment groups receiving 0, 2, 4, or 6 mg Zn-CS/kg body weight per day for 90 days. Growth performance, nutrient digestibility, plasma biochemical parameters, liver function enzymes, serum hormones, antioxidant indices, fec...
Effects of supplementing concentrates with different energy levels on growth performance, blood indicators, and fecal microbiota of Mongolian horses in late gestation. Horse breeding and management have a globally extensive history, with horses serving vital roles in transportation, warfare, agriculture, and cultural activities. In China, particularly in Inner Mongolia, the horse industry faces challenges such as harsh winters and limited nutritional research for local breeds, especially during late gestation. This critical period is essential for fetal development and maternal health, and inadequate nutrition can lead to complications like nutritional abortions and poor foal growth. Similar issues exist worldwide in cold climates or regions with seasonal pa...
Evaluating hormonal levels in plasma and saliva as temperament indicators in horses. Assessing the temperament of horses is important to ensure their welfare, management, and safety. This study evaluated the plasma-saliva relationships for cortisol, oxytocin, and serotonin, and explored their associations with temperament traits in 40 horses using enzyme-linked immunosorbent assays. Significant correlations were observed between cortisol (r = 0.441) and oxytocin (r = 0.587) concentrations, whereas serotonin showed a weak correlation (r = 0.133). Bland-Altman analysis revealed acceptable agreement for cortisol but substantial individual variability for oxytocin, despite its str...
The Equine Reproductive Microbiota: Composition, Dynamics, Dysbiosis, and Implications for Fertility in Mares and Stallions. The equine reproductive microbiota has emerged as an important factor influencing reproductive health and fertility in both mares and stallions. Traditionally, the equine uterus was considered sterile, and microbial presence was interpreted primarily in the context of infection. However, sequencing-based studies have demonstrated that the reproductive tract, including the uterus and semen, contains detectable microbial communities or microbial DNA signatures, challenging this traditional paradigm. In mares, the vaginal microbiota is consistently dominated by , , , and , whereas the uterine env...
Beyond cortisol: evaluating serotonin, brain-derived neurotrophic factor, and oxytocin as indicators of equine welfare across three training regimens. BACKGROUND: Quantifying the balance between workload, stress, and recovery across equine disciplines remains challenging, and no single circulating biomarker provides a definitive index of welfare-related stress load. Cortisol, oxytocin, serotonin (5-hydroxytryptamine, 5-HT), and brain-derived neurotrophic factor (BDNF) may provide complementary information on endocrine and neuromodulatory responses. METHODS: Paired serum samples were collected from show jumping horses (n = 17) before and after competition, racehorses (n = 43 enrolled, n = 41 analyzed) before and after sessions at ...
The Effects of L-Citrulline on the Apparent Digestion and Metabolism of Nutrients, Blood Hormone Levels, Amino Acid Metabolism and the Diversity of Faecal Microbiota in Mares in the Later Stage of Pregnancy. This study aimed to investigate the impact of dietary L-citrulline supplementation on the health of mares during late gestation. Thirty-two healthy mares in late pregnancy were randomly assigned to four groups: a control group (CON, 0 g/d) and three treatment groups receiving 15, 30, and 45 g/d/head of L-citrulline, respectively. The trial spanned 72 days, including a 12-day adaptation phase and a 60-day formal feeding period. A fixed daily feeding amount of 11.2 kg/head was provided, ensuring complete consumption and consistent dry matter intake across all groups. Results demonstrated that su...
A scoping review of neuroendocrinological biomarkers with potential for use in equine behavioral medicine practice. BACKGROUND: Equine emotion and welfare are of increasing concern to the general public, equestrians and researchers. There is growing demand for more objective indicators of specific emotional states, as well as the well-being of horses in a variety of contexts. Such markers may also assist in both the diagnosis and selection of biological therapies within equine behavioral medicine. RESULTS: This scoping review summarizes and appraises the published findings relating to relevant biological measurements from the last two decades. Particular attention is given to methodological, subject-related...
Transference of diagnostic thresholds for equine plasma ACTH from the Immulite 1000 to Immulite 2000XPi and Tosoh AIA systems. Diagnosis of PPID in horses relies on measurement of plasma adrenocorticotrophic hormone (ACTH) concentrations. Because measured ACTH values vary between analytical platforms, and published diagnostic thresholds apply almost exclusively to the Immulite 1000 chemiluminescent assay (CLA), there is a need for validated, assay-specific thresholds for contemporary analysers. This study aimed to transfer and validate established Immulite 1000 diagnostic thresholds for plasma ACTH to three contemporary systems (Immulite 2000XPi, Tosoh AIA-900, and Tosoh AIA-360) using the Clinical Laboratory Standard...
Relationship between the estrus duration in cyclic mares and the endometrial gene expression on day 7 post-ovulation. This study investigated the effect of estrus duration on endometrial gene expression in cyclic mares on Day 7 post-ovulation, focusing on transcripts relevant to early pregnancy: uterocalin (P19), insulin-like growth factor 1 (IGF1), fibroblast growth factor 2 (FGF2), progesterone receptor (PGR), and estrogen receptor alpha (ESR1). Twelve mares were classified according to the estrus duration in long estrus (LE, ≥ 3 days of endometrial edema; n = 7) or short estrus (SE, 0.1). ESR1 expression was undetectable in all samples. These findings suggest that prolonged estrus enhances P19 e...
Effect of season of collection and storage method on equine hair cortisol concentration. Serum cortisol concentration is one of the common physiological reference standards for evaluating stress in animals. However, it only evaluates acute stress at the moment of blood collection. Hair cortisol concentrations (HCC) could provide a method for assessing cortisol concentrations over extended periods of time, giving an estimate of chronic stress levels. In this case, high HCC results could help equine managers alter management practices to improve welfare. Objective: To evaluate the effect of the month of collection and the storage method on equine HCC. Methods: Tail hair was collecte...
