Results of this study were presented in abstract form at the 21st Annual Meeting of the American College of Veterinary Internal Medicine.
Corresponding author: Babetta A. Breuhaus, DVM, PhD, DACVIM, Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606; e-mails: email@example.com, firstname.lastname@example.org.
Background: This study was performed to determine whether anhidrotic horses have altered thyroid function compared with horses that sweat normally.
Hypothesis: Anhidrotic horses have normal thyroid function.
Animals: Ten client-owned horses with clinical signs of anhidrosis were paired with 10 horses living in the same environment that had normal sweat production.
Methods: Horses were diagnosed as having normal sweat production or being anhidrotic based on responses to intradermal injections of terbutaline and physiologic responses to lunging exercise. Control horses were selected from the same environment and matched as closely as possible to anhidrotic horses in terms of age, sex, breed, and athletic condition. Thyrotropin-releasing hormone (TRH) stimulation tests were performed in both horses at the same time, once in the summer or fall, and once again in winter.
Results: Anhidrotic horses produced less sweat in response to intradermal injections of terbutaline and exercise than did control horses. They also had greater increases in body temperature and respiratory rate in response to exercise. Resting concentrations of thyroid hormones and thyroid-stimulating hormone (TSH) were not different between anhidrotic and control horses. Thyroid hormone responses to TRH also were not different between the 2 groups of horses. However, anhidrotic horses had a significantly different TSH response to TRH compared with control horses, particularly in the winter.
Conclusions and Clinical Importance: The biologic relevance of the altered TSH response to TRH in anhidrotic horses is uncertain, considering that TSH concentrations remained within previously reported normal ranges and thyroid hormone responses were not different between anhidrotic and control horses.
Thermoregulation is an important component of exercise performance that is sometimes overlooked. Muscles convert approximately 20% of the chemical energy they burn into mechanical energy for work, with the remainder being released as heat. This heat must be dissipated from the body in order to maintain normal body temperature. In exercising horses, approximately 15–25% of heat loss occurs through the respiratory tract, whereas as much as 65% of heat loss is accomplished by sweating.1
Anhidrosis is a condition of adult horses characterized by decreased ability or inability to sweat in response to appropriate stimuli.2,3 The condition initially was described in horses taken from temperate climates to hot, humid environments for racing purposes.4 Since then, anhidrosis has been reported in horses raised in or native to hot, humid environments, in addition to those that have been imported.5–7 In the United States, anhidrosis is most prevalent in the Gulf Coast states. The prevalence of anhidrosis also is higher in horses in training than in sedentary horses; it has been estimated that 20% of racehorses in the southern United States are affected.5,6 Similar data are not available for horses used for other athletic endeavors such as dressage, hunter/jumper, and pleasure, but anhidrosis is not limited to horses that race.5 Inadequate sweat production, particularly in a hot environment, interferes with body temperature regulation and has a profound detrimental effect on a horse's ability to work and compete athletically.
Clinical signs of anhidrosis include exercise intolerance, tachypnea, increased body temperature, slower rate of recovery of respiratory rate after exercise, and absence or decreased amount of sweat. Some anhidrotic or hypohidrotic horses have areas of the body that retain the ability to sweat to varying degrees (eg, under the mane or halter or in the inguinal, perineal, or axillary regions).2 Anhidrosis is suspected based on clinical signs and can be confirmed by observing failure of a horse to sweat in response to injection of a β2-adrenergic agonist into the skin.2,4,8
The cause of anhidrosis in horses is unknown. As recently reviewed,3 it is thought to be caused by a gradual decrease in the secretory response of sweat gland cells to appropriate stimuli,8–11 eventually leading to gland degeneration.12,13 Decreased sweat gland function in anhidrotic horses is not associated with changes in gland innervation12 or decreased concentrations of circulating agonists.9,14 Equine sweat glands are stimulated to secrete primarily by activation of β2-adrenergic receptors,15–17 although responses to purinergic agonists also have been demonstrated.18 A recent study showed that sweat gland cells from anhidrotic horses, grown in culture, had significantly smaller secretory responses to purinergic agonists or isoprenaline, a β-adrenergic agonist, compared with sweat gland cells cultured from freely sweating horses.11 It has been proposed that anhidrosis results from desensitization or downregulation of β2-adrenergic receptors,3 but to date this hypothesis has not been confirmed and the mechanism is unknown.
Hypothyroidism has been suggested as a cause of anhidrosis in horses, perhaps because treatment with iodinated casein has been reported to ameliorate clinical signs.19 Thyroid hormones can modulate adrenergic receptor function in other species,20–23 and therefore could alter β-adrenergic receptor function in anhidrotic horses. Additional research is required to establish the role of thyroid function in anhidrotic horses.2 The study reported here was performed to compare thyroid function between anhidrotic horses and horses that sweat normally.
Materials and Methods
Anhidrotic horses were recruited for this study with the help of referring veterinarians and clients of the North Carolina State University-Veterinary Teaching Hospital (NCSU-VTH). Horses were confirmed to be anhidrotic, hypohidrotic, or normal by means of history, physical examination, respiratory rate, and sweat responses to lunging exercise at a trot for 30 minutes, and by response to intradermal injections of terbutaline sulfate. Nineteen horses with a history of anhidrosis were evaluated for possible inclusion in the study. Ten anhidrotic or hypohidrotic horses (3 mares and 7 geldings, ranging in age from 5 to 25 years) were enrolled in the study. Six of these horses were classified as completely anhidrotic (ie, they produced no more than a trace of sweat at the highest concentrations of terbutaline and did not sweat at all during lunging exercise). The remaining 4 horses were classified as hypohidrotic. These horses sweated mildly during lunging (eg, in the flank, just behind the elbow, or at the base of the mane) and they produced small amounts of sweat at higher concentrations of terbutaline only. Once the anhidrotic or hypohidrotic horses were identified, a control horse was selected from the same farm, unless one was not available. The control horses (5 mares and 5 geldings, ranging in age from 3 to 16 years) were matched as closely as possible to the anhidrotic horses, in terms of training intensity, breed, sex and age. All except 1 of the horses were client owned. Owners of 2 of the anhidrotic horses did not own or have access to another horse. The control horse for 1 of these 2 horses was selected from the North Carolina State University teaching herd and the 2nd control horse was volunteered by an owner at 1 of the other barns. Control horses revealed normal sweat responses to lunging exercise and intradermal injections of terbutaline. A thyrotropin-releasing hormone (TRH) stimulation test was performed on each anhidrotic/control pair at the same time in their home environments, once during warm weather when the anhidrotic horses were having problems with body temperature regulation (July to October), and once again in January or February, when ambient temperatures were low enough that sweat production was unnecessary for body temperature regulation. One pair of horses was studied in the summer only, because the owner had moved away by the winter time. Study horses lived in North Carolina, South Carolina, and Georgia.
The study design required horses not receive medications for at least 4 weeks before enrollment in the study. However, at the time they were identified for inclusion in the study, a few horses (2 completely anhidrotic and 1 hypohidrotic horse) were already receiving a producta that was supposed to increase sweat production. Because owners were reluctant to discontinue administration of this product, horses that were already receiving it were admitted to the study if results of the skin testing and lunge testing indicated that they still did not sweat normally.
Five 10-fold serial dilutions of terbutaline sulfateb were made from the initial commercially supplied concentration of 1 mg/mL. A 0.1 mL aliquot of each of the resulting 6 dilutions (1 mg/dL to 0.00001 mg/mL) was injected intradermally along the left side of each horse's neck. The most dilute concentration (0.00001 mg/mL) was injected 1st, closest to the head. Increasingly more concentrated aliquots were injected approximately 8 cm apart, in a straight line going toward the shoulder blade. Time of injection was noted (total time to inject all 6 dilutions was <2 minutes). For each injection site, the time that sweat 1st appeared also was recorded. Thirty minutes after injection of the last aliquot, the dimensions of the wet hair around each injection site were measured with a ruler, and the area of sweat estimated as an ellipse, a rectangle, or as a combination of these shapes. Normal horses started to sweat at all terbutaline dilutions, with the amount of sweat produced increasing as the concentration of terbutaline increased. In some normal horses, the amount of sweat produced at the most dilute concentration was small enough that the sweat had dried by 30 minutes after injection. In contrast, completely anhidrotic horses failed to sweat at any terbutaline dilution. Horses that failed to sweat at the more dilute concentrations of terbutaline but produced small amounts of sweat at the higher terbutaline concentrations were classified as hypohidrotic. These horses were enrolled in the study only if they also failed to sweat normally while lunging at a trot in warm weather.
Response to Lunging Exercise
Horses were lunged in a circle at a trot outdoors, 15 minutes in one direction, and then 15 minutes in the other direction. They were observed both during exercise and for 30 minutes after conclusion of the exercise. Control horses sweated along the neck, the trunk, over the back, and between the hind legs. Anhidrotic and hypohidrotic horses did not sweat at all, or produced only small amounts of sweat in the flank area, in the girth area, or under the mane. Rectal temperature, heart rate, and respiratory rate were recorded just before the start of exercise. Heart rate and respiratory rate were taken immediately and 2, 5, 10, 20, and 30 minutes after exercise was finished. Rectal temperature was taken 5, 10, 20, and 30 minutes postexercise. After the 30 minute postlunging measurements were obtained, horses were hosed with water to facilitate cooling, especially those that did not sweat. Average ambient temperature and humidity while horses were lunging were 84.5 ± 5.9°F and 62.2 ± 5.4%, respectively.
Horses were confined to their stalls for the duration of the TRH stimulation tests (4 hours). They were fed their normal diets before the tests, and were allowed free access to water and hay during the study period. Control blood was withdrawn by venipuncture and horses were given 1 mg TRHc between 9:00 am and 12:00 pm, usually around 10:00 am. Additional blood samples were drawn 15, 30, 45, 60, 120, and 240 minutes after administration of TRH. Blood was allowed to clot at ambient temperature and then serum was removed and either frozen immediately or stored on ice until returned to the laboratory. Samples were stored at −70 °C until assay. All samples were assayed for thyroid-stimulating hormone (TSH). Thyroid hormone concentrations were measured at 0, 60, 120, and 240 minutes.
Serum total and free thyroxine (T4) and free tri-iodothyronine (T3) concentrations were measured with commercially available radioimmunoassay (RIA) kits.d,e,f Total T3 and TSH were measured by RIA by described procedures24,25 and modifications.26 All assays have been previously validated for use in the horse.27–29 Total T4 assay sensitivity was 3 nmol/L; normal range in adult euthryoid horses, 6–46 nmol/L. Free T4D assay sensitivity was 1.8 pmol/L; normal range in adult euthyroid horses, 7–47 pmol/L. Free T3 assay sensitivity was 0.1 pmol/L; normal range in adult euthyroid horses, 1.7–5.2 pmol/L. Total T3 assay sensitivity was 0.3 nmol/L, normal range in adult euthyroid horses, 0.7–2.5 nmol/L. TSH assay sensitivity was 0.02 ng/mL, normal range in adult euthryoid horses, 0.02–0.97 ng/mL.
Rectal temperature, heart rate, and respiratory rate responses to lunging at a trot for 30 minutes were compared between anhidrotic horses and control horses by analysis of variance (ANOVA) for repeated measures (time). Resting values, peak changes, and peak differences were compared by a t-test.30,g Time from injection of terbutaline until the 1st appearance of sweat was compared between anhidrotic and control horses by the Kruskal-Wallis test at the 2 most concentrated dilutions of terbutaline only.30,g Area of sweat production at each dilution of terbutaline also was compared between anhidrotic and control horses by the Kruskal-Wallis test. Mean concentrations of TSH, total and free T4, and total and free T3 in pre-TRH (resting) serum samples of anhidrotic and control horses were compared by a t-test.30,g TSH and thyroid hormone responses to TRH were assessed by ANOVA for repeated measures (time), in a 2 × 2 (sweat status, season) factorial design. Where indicated, posthoc tests were performed by the Bonferroni adjustment.30,g Significance was set at P < .05. Except as noted, data are expressed as the mean ± SD.
Few horses had measurable areas of sweat production 30 minutes after injection of the most dilute (0.00001 mg/mL) concentration of terbutaline. Control horses produced more sweat than anhidrotic and hypohidrotic horses at all other concentrations (Table 1). Control horses also began to sweat more quickly after terbutaline injection than did anhidrotic and hypohidrotic horses. The elapsed time could only be compared at the 2 highest concentrations of terbutaline, because the anhidrotic horses did not sweat at the lower concentrations. The time between injection and the 1st appearance of sweat was >2 times greater in anhidrotic and hypohidrotic horses compared with control horses (Table 2).
Table 1. Area (in cm2) of sweat production for each concentration of terbutaline sulfate (0.1 mL of 0.00001 to 1 mg/mL injected intradermally) in anhidrotic and control horses.
Terbutaline Concentration (mg/mL)
Area (cm2) of Sweat Production (mean ± SD)
Control Horses (n = 10)
Anhidrotic Horses (n = 10)
P < 0.05 anhidrotic horses compared with control horses.
Pre-exercise heart rates and heart rate responses to exercise were not different between anhidrotic horses and horses that sweated normally (Fig 1). Pre-exercise rectal temperatures also were not different between anhidrotic horses and horses that sweated normally, but postexercise temperatures and the differences between peak rectal temperatures and pre-exercise temperatures were significantly greater in anhidrotic horses compared with controls (peak differences 2.1 + 0.6 versus 1.1 + 0.5 °C, respectively, P= .002) (Fig 2).
Resting respiratory rate was not significantly different between anhidrotic horses and horses that sweated normally. However, peak respiratory rates, differences between peak rates and pre-exercise rates, and overall respiratory rate responses to lunging exercise were significantly greater in anhidrotic horses compared with controls (Fig 3). The most dramatic increase in respiratory rate occurred in 1 horse whose resting respiratory rate was 60/minute. In this horse, respiratory rate was 192/minute for the 1st 10 minutes after exercise was discontinued. Respiratory rate was still 174/minute 20 minutes after the end of exercise. It decreased to 120/minute after the horse was hosed with cold water. In all horses that could sweat normally, respiratory rate had returned to the pre-exercise value by 30 minutes after the end of trotting exercise. Respiratory rate had not returned to the pre-exercise rate in any of the anhidrotic or hypohydrotic horses by 30 minutes after the end of exercise.
There was no significant effect of season (summer versus winter) or sweat status (control versus anhidrotic) for baseline (pre-TRH) TSH concentrations. However, the overall TSH response to TRH administration (over the entire 4-hour period) was significantly greater in anhidrotic horses compared with horses that could sweat normally (P= .036, Fig 4). There was also a significant interaction between time and sweat status (P= .013). The increase in TSH was prolonged in anhidrotic horses, peaking at 2 hours post-TRH administration, instead of at 1 hour. If the data are further analyzed by season, the differences between anhidrotic and control horses are lost during summer, but remain significant during winter (P= .029 for sweat status and .012 for time, sweat status interaction). There was no significant effect of season (summer versus winter) or sweat status (control versus anhidrotic) for any of the baseline thyroid hormone concentrations, nor for the thyroid hormone responses to TRH administration (Fig 5).
Ours is the 1st study to critically analyze thyroid function in anhidrotic horses. Despite a previous report that administration of iodinated casein to anhidrotic horses improved their ability to sweat,19 anhidrotic horses appear to have mostly normal thyroid function. The current findings are consistent with a previous study in which resting concentrations of T4 and T3 were normal in 5 anhidrotic horses.5 However, in that study TRH stimulation tests were not performed, nor was TSH measured.
Although TSH responses to TRH were statistically different in anhidrotic horses compared with horses with normal sweat production, the biological relevance of this difference is uncertain. TSH responses to TRH in all horses in the present study were within 1 SD of normal ranges reported previously.29,31,32 In addition, there was no significant difference in TSH response to TRH when anhidrotic horses were compared with control horses in summer only, the time of year when anhidrotic horses reveal clinical signs. Still, it does appear that the TSH response to TRH in anhidrotic horses was different from control horses in this study, and from horses in previous studies, in that TSH had not started to decrease by 2 hours after TRH injection in anhidrotic horses. In previous studies,29,31,32 and in control horses in the present study, TSH peaked 1 hour after TRH administration. The clinical relevance of this prolonged TSH response to TRH in anhidrotic horses is unknown.
In humans, increased TSH concentrations in conjunction with normal thyroid hormone concentrations is suggestive of subclinical hypothyroidism.33,34 However, the presence of subclinical hypothyroidism in anhidrotic horses in this study seems unlikely, because many had been anhidrotic for years.
Comparison of thyroid hormone and TSH responses to TRH between severely anhidrotic horses and hypohidrotic horses in this study failed to establish a trend for a more profound decrease in thyroid function in more severely affected horses (data not shown). Thus, it is unlikely that studying additional horses or including only severely affected horses would have altered the overall results of the study.
In conclusion, anhidrotic horses in this study had mostly normal thyroid function, with the exception of their TSH responses to TRH. The clinical relevance of these results is uncertain, and, without additional studies, should not be interpreted to mean that hypothyroidism is a cause of anhidrosis in horses. Any improvement in sweat production after administration of iodinated casein or thyroid hormones to anhidrotic horses potentially could be pharmacologic rather than physiologic. For example, thyroid hormone administration to normal rats increased the numbers of β-adrenergic receptors in heart muscle, with no alteration in receptor affinity.20 Erythrocytes from turkeys made hyperthyroid by administration of T4 generated more cAMP in response to isoproterenol than did normal cells, without a concomitant change in β-adrenergic receptor numbers.22 Thus, administration of thyroid hormones to anhidrotic horses could potentially augment β-adrenergic receptor numbers or sensitivity on equine sweat gland cells or amplify signals distal to those receptors, thereby potentiating the β-adrenergic-mediated sweat response, regardless of whether anhidrotic horses are euthyroid or subclinically hypothyroid.
aOne AC; MPCO, Phoenix, AZ
bTerbutaline sulfate; Novartis Pharmaceuticals Corporation, East Hanover, NJ
cpGLU-HIS-PRO amide; Sigma Chemical Co, St Louis, MO
dMagic T4; Ciba Corning Diagnostics, East Walpole, MA
eFree T4 by equilibrium dialysis; Nichols Institute Diagnostics, San Juan Capistrano, CA
fMagic fT3; Ciba Corning Diagnostics
gSYSTAT, Evanston, IL
The author thanks Drs Kent Refsal and Raymond Nachreiner, Michigan State University, for thyroid hormone assays; Dr Don Thompson, Louisiana State University, for TSH assays; and Mr D. Heath LaFevers for technical assistance.
This study was supported by funds from USA Equestrian, 4047 Iron Works Parkway, Lexington, KY 40511-8483 and the Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606