Serum thyroid hormone, insulin, glucose, triglycerides and protein concentrations in normal horses: Association with topical dexamethasone usage

Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals

Equine thyroid disorders pose a diagnostic challenge in clinical practice because of the effects of nonthyroidal factors on the hypothalamic-pituitary-thyroid axis, and the horse's ability to tolerate wide fluctuations in thyroid hormone concentrations and survive without a thyroid gland. While benign thyroid tumours are common in older horses, other disorders like primary hypothyroidism or hyperthyroidism in adult horses and congenital hypothyroidism in foals are rare. There is a common misunderstanding regarding hypothyroidism in adult horses, especially when associated with the clinical profile of obesity, lethargy, and poor performance observed in dogs and humans. Low blood thyroid hormone concentrations are often detected in horses as a secondary response to metabolic and disease states, including with the nonthyroidal illness syndrome; however, it is important to note that low thyroid hormone concentrations in these cases do not necessarily indicate hypothyroidism. Assessing equine thyroid function involves measuring thyroid hormone concentrations, including total and free fractions of thyroxine (T4) and triiodothyronine (T3); however, interpreting these results can be challenging due to the pulsatile secretion of thyroid hormones and the many factors that can affect their concentrations. Dynamic testing, such as the thyrotropin-releasing hormone stimulation test, can help assess the thyroid gland response to stimulation. Although true hypothyroidism is extremely rare, thyroid hormone supplementation is commonly used in equine practice to help manage obesity and poor performance. This review focuses on thyroid gland pathophysiology in adult horses and foals, interpretation of blood thyroid hormone concentrations, and evaluation of horses with thyroid disorders. It also discusses the use of T4 supplementation in equine practice.

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.

Systemic absortion and adverse effects of topical ocular use of ketorolac tromethamine and sodium diclofenac in New Zealand rabbits for 90 days

ABSTRACT The effect of the systemic absorption of 0.1% diclofenac sodium (DS) eyedrop was compared to that of 0.5% ketorolac tromethamine (KT) in female New Zealand white rabbits treated on both eyes three times a day for 90 days. The rabbits were divided in three groups of six animals (n= 18): KT group, DS group, and control (Co) group, in which saline (0.9% NaCl) solution was instilled. Water and food consumption were measured daily, clinical examination was performed weekly, and blood samples were collected every 30 days for laboratory examination. The plasma was analyzed for the presence of KT and DS by solid-phase extraction (SPE) associated with mass spectrometry (MS). Systemic absorption of these drugs was confirmed by SPE-MS, allowing their separation and identification in the plasma. At the end of the treatment, the animals were euthanized and necropsied, and no macroscopic or microscopic changes were found. This observation confirmed the laboratory results, which were within normal reference standards for the species. According to the results obtained, it can be concluded that treatment with eyedrops containing KT and DS for 90 days in healthy rabbits does not cause adverse systemic effects.

Metabolic and Endocrine Disorders in Donkeys

The donkey evolved under harsh and arid environmental conditions, developing unique energy-efficiency traits, with an efficiency to rapidly mobilize fat in situations of increased energy demands or when food is scarce. This evolution has led to an inherent predisposition of donkeys to obesity, dyslipidemias, insulin dysregulation/metabolic syndrome, pituitary pars intermedia dysfunction, and endocrinopathic laminitis. Marked differences have been described in hormone dynamics and testing protocols for the diagnosis of these endocrine and metabolic diseases in donkeys compared with horses, underlining the necessity of a species-specific approach in order to avoid misdiagnosis, unnecessary or inadequate treatments, and additional costs.

Donkey Internal Medicine—Part I: Metabolic, Endocrine, and Alimentary Tract Disturbances

Metabolic and endocrine disturbances are common in donkeys. This species has an inherent ability to thrive with limited and poor-quality roughage. Donkeys are extremely efficient in energy storage and mobilization, which predisposes to hyperlipemia, obesity, and metabolic syndrome. The prevalence of dyslipidemias is higher in donkeys than other equids, which is more evident under stressful conditions. Diagnosis of endocrine and metabolic disorders in donkeys should be based on species-specific information considering that differences in a multitude of variables compared with horses have been demonstrated. Protocols to assess endocrine disorders (e.g., pituitary pars intermedia dysfunction, metabolic syndrome, and thyroid illness) are unavailable, and extrapolation from horse data can be misleading. Treatment guidelines for these conditions in donkeys are currently not reported. On the other hand, the typical stoic and hardy behavior of donkeys can hinder prompt diagnosis of gastrointestinal problems, specifically colic, which is commonly caused by dental issues in this species. Moreover, subclinical gastric ulcer syndrome appears to be a common pathology in this species, especially in working donkeys.

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Donkeys are different to horses.

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Numerous physiological and clinic-pathologic idiosyncrasies are reported in horses.

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Data published for horses should not be extrapolated for donkeys.

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Specific reference ranges, doses, and protocols have to be used for donkeys.

The contribution of total and free iodothyronines to welfare maintenance and management stress coping in Ruminants and Equines: Physiological ranges and reference values

In order to acquire a pattern of thyroid involvement in welfare maintenance in Ruminants and Equines, this review summarizes data concerning the reference values of total and free iodothyronines and their modifications in physiological conditions and in different management conditions (pregnancy, lactation, weaning, growth, isolation, restraint, shearing, confinement and transportation). Thyroidal and extrathyroidal tissues efficiently respond to management practices, giving a differentiated contribution to circulating iodothyronine changes. The hormonal response could be mainly attributed to the intracellular deiodination of T₄ to T_3. Triiodothyronine (T₃) and free iodothyronines (fT₃ and fT₄) result more responsive to management stress, showing different pattern with species and to various conditions, as to environmental conditions in which activities are performed. Intrinsic seasonal changes of iodothyronines and a significant pregnancy effect for T₃ were recorded in mares. Higher, although not significant, T₃ and T₄ concentrations in barren than pregnant mares were observed in donkeys. A positive significant correlation between T₃ and T₄ was described only in pregnant donkeys. Moreover, a significant effect of season on T₃ and fT₃ changes was observed both in pregnant and barren donkeys. A significant lactating effect compared with nonlactating stage for T₃ and T₄ was recorded in mares. In growing foals, body weight (BW) and age were positively correlated with T₃ and negatively correlated with T₄, fT₄ and fT₃. Weaning effects were shown for T₃ and fT₄ concentrations, indicating that weaning represents a severe stress and the presence of conspecific does not reduce psychological stress in this phase. Lambs showed significant decreased T₃ and elevated T₄ concentrations two weeks after weaning, with higher concentrations in both males and females compared to 24 h. Significant positive correlations were observed between BW and T₄, fT₃ and fT₄ concentrations in lambs. A T₃ decrease was detected after isolation, such as induced by confinement and weaning in lambs. Higher T₃ concentration after restraint and shearing than after isolation and significant increases in T₄, fT₃ and fT₄ values after restraint and shearing were recorded. The basal concentrations of fT₃ in both the inexperienced and experienced transported horses were significantly higher than in untransported experienced horses. Moreover, increases of T_3, T₄ and fT₄ after short road transportation, and significant correlations between T₃ and rectal temperature (RT), body weight (BW) and heart rate (HR), confirmed their important role in coping strategy. Thyroid responsiveness to short transport is similar in domestic donkeys and horses, with a preferential release of T₃ in horses. A greatest and constant release of T₃ and T₄, although differentiated, after simulated transportation and after conventional transport of horses confirmed that the degree of stress induced by confinement and additional stressful stimuli associated to road transportation could differently influence the iodothyronine release. Temperamental Limousin young beef bulls showed lower T₄ and fT₄ concentrations after prolonged transportation than calm subjects, and a concomitant decrease of circulating ACTH, cortisol, T₃ and fT₃ concentrations, probably induced by down regulation of HPA axis and cortisol negative feedback. These data reinforce the importance of taking into account the evaluation of iodothyronines, and notably of T₃, as markers of welfare and stress and their role in ensuring energy homeostasis and productive and reproductive performances in Ruminants and Equines.

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.

Nonthyroidal illness syndrome in adult horses

BACKGROUND

This study was performed to determine whether sick horses have thyroid hormone (TH) alterations similar to those observed in nonthyroidal illness syndrome in other species.

HYPOTHESIS

Horses suffering from systemic diseases have decreased THs and inappropriately low thyroid-stimulating hormone (TSH).

ANIMALS

Seventy-one clinically normal horses; 380 hospitalized horses.

METHODS

Total thyroxine (TT4), free thyroxine by equilibrium dialysis (fT4D), total triiodothyronine (TT3), free triiodothyronine (fT3), and TSH were measured in normal and hospitalized horses. Disease severity was categorized as mild, moderate, or severe by both subjective and objective criteria.

RESULTS

Negative correlations existed between all THs, except TSH, and objective illness severity scores. These scores also increased with each subjective disease severity category. TT3 and fT3 were decreased with mild disease. TT3 progressively decreased more with moderate and severe disease. TT4 and fT4D remained normal with mild disease, but decreased progressively with disease severity. TSH increased with mild disease, but remained normal with moderate or severe disease. Horses that died or were euthanized had lower concentrations of all THs, except TSH, when compared with those that lived. In horses that received >3 doses of NSAIDs, corticosteroids, or heparin compared to 0-3 doses, TT3 and TT4 were decreased, whereas fT4D and TSH remained normal. There were minimal TH changes in horses that were not eating.

CONCLUSIONS AND CLINICAL IMPORTANCE

Thyroid hormones decrease in horses with systemic disease. TT3 decreases first, followed by TT4 and fT4D. TSH fails to increase proportionally to the changes in THs, indicating hypothalamic-pituitary axis dysregulation. NSAIDs, corticosteroids, heparin, and fasting have less effect on THs compared with disease severity.

Thyroid hormone concentrations differ between donkeys and horses

REASONS FOR PERFORMING STUDY

Reference intervals for thyroid hormones (TH) concentrations have not been previously established for donkeys, leading to potential misdiagnosis of thyroid disease.

OBJECTIVES

To determine the normal values of TH in healthy adult donkeys and compare them to TH values from healthy adult horses.

METHODS

Thirty-eight healthy Andalusian donkeys and 19 healthy Andalusian horses from 2 different farms were used. Donkeys were divided into 3 age groups: <5, 5-10 and >11 years and into 2 gender groups. Serum concentrations of fT3, tT3, rT3, fT4 and tT4 were quantified by radioimmunoassay. All blood samples were collected the same day in the morning. None of the animals had received any treatment for 30 days prior to sampling or had any history of disease. Both farms were in close proximity and under similar management. Differences between groups were determined using a one-way ANOVA analysis followed by Fisher's LSD test. P<0.05 was considered significant.

RESULTS

Serum TH concentrations were higher in donkeys than in horses (P<0.01). Donkeys <5 years had higher serum rT3, fT4 and tT4 concentrations than donkeys >5 years (P<0.05). Furthermore, older donkeys (>11 years) had lower serum fT3 and tT3 concentrations than younger donkeys' groups (<5 and 5-10 years, P<0.05). TH concentrations were not different between genders (fT3: P = 0.06; tT3: P = 0.08; rT3: P = 0.15; fT4: P = 0.89; and tT4: P = 0.19).

CONCLUSIONS

Thyroid hormone concentrations are different between healthy adult donkeys and horses.

POTENTIAL RELEVANCE

Establishing species-specific TH reference ranges is important when evaluating clinicopathologic data in equids in order to avoid the misdiagnosis of thyroid gland dysfunction. Further studies to elucidate the physiological mechanisms leading to these differences are warranted.