Thyroid disorders in horses present a significant diagnostic challenge due to the complex interplay of factors influencing thyroid hormone concentrations.1 Serum concentrations of T4 and T3 are not always indicative of thyroid gland function or the integrity of the hypothalamic-pituitary-thyroid axis.2 Variations in T3 and T4 concentrations can result from a multitude of factors including age, diurnal rhythms, diet, climate, reproductive status, and physical training.3–8 Additionally, nonthyroidal illnesses and the administration of various medications can also lead to decreased thyroid hormone concentrations, complicating the diagnosis further.9–11 A nonthyroidal illness syndrome has been documented in horses, and one condition that can trigger this syndrome is pituitary pars intermedia dysfunction (PPID).12–14
Pituitary pars intermedia dysfunction is the most prevalent endocrine disorder in older horses, affecting up to 20% of horses and ponies over 15 years of age.15 This progressive neurodegenerative condition results from the loss of dopaminergic inhibition of the pars intermedia in the pituitary gland, leading to the overproduction of pro-opiomelanocortin–related hormones.16 Horses with PPID generally have lower resting thyroid hormone concentrations when compared to age-matched healthy controls; however, despite this reduction, the thyroid gland retains the ability to function normally under stimulation, as evidenced by thyrotropin-releasing hormone (TRH) stimulation tests.14 This suggests that the lower concentrations of thyroid hormones in horses with PPID would not be due to intrinsic thyroid dysfunction but might be secondary to the systemic effects of the disease or the influence of medications.
One such medication is pergolide mesylate, a dopamine receptor agonist widely used in the management of PPID.17 Pergolide is highly protein bound, with over 90% of the drug being bound to plasma proteins.18 Like what is described with phenylbutazone, this high degree of protein binding might interfere with the binding of thyroid hormones to their respective proteins, potentially leading to increased urinary loss and reduced circulating concentrations of T3 and T4.11 Consequently, pergolide administration could contribute to lower thyroid hormone concentrations in horses with PPID, further complicating the assessment of thyroid function in these animals.
Given the complex interactions between PPID, thyroid hormone concentrations, and the effects of pergolide, this study aims to clarify the impact of pergolide administration on thyroid hormone concentrations and overall thyroid function in horses. The hypothesis is that while pergolide administration might lower resting T3 and T4 concentrations, it does not impair the thyroid gland’s ability to respond to TRH stimulation, thus preserving its functional capacity.
Methods
Design
To evaluate the effect of pergolide on thyroid hormone concentration and thyroid function, an analytic, observational, cohort study design was elected. Horses were tested at baseline (day 0), treated with pergolide for 5 days (days 1 to 5), and followed for another 6 days (days 6 to 11).19 On days 0 and 6, a TRH stimulation test was performed to fully assess thyroid function. A power calculation for sample size was conducted using data from Ramirez et al.11 The ability to detect a difference between 5 and 15 nmol/L in total T4 (tT4) with a power of 0.8 and an α = 0.05 yielded a sample size of 6. This study was approved by the IACUC (protocol No. 1222002329).
Horses
Six light-breed adult horses from the Institutional herd were used. Horses were determined to be clinically healthy based on medical records and physical examination. Horses median age was 19 (17 to 24) years and included 4 mares and 2 geldings. Weights ranged from 530 to 599 kg (median, 566 kg). Breeds included Standardbred (n = 3), Quarter Horse (1), Saddlebred (1), and Tennessee Walking Horse (1). None of the mares were pregnant or lactating at the time of the study, and none of the horses had received any medications for 4 weeks before sampling. Although all the horses had ACTH concentrations within normal limits at baseline, 2 horses had ACTH concentrations consistent with PPID 10 min after a TRH stimulation (Supplementary Material S1). Horses were housed in individual stalls and fed free choice grass hay and a complete senior pelleted feed (Purina Animal Nutrition LLC) for the duration of the study. Water was available for ad libitum intake.
Resting thyroid hormone concentrations
After acclimating overnight, each horse had blood collected for a complete thyroid panel (total T3 [tT3], tT4, and free T4 [fT4]), and a TRH stimulation test was performed. Blood was collected by jugular venipuncture into an evacuated plain tube. Within 1 hour of blood collection, samples were centrifuged at 1,500 X g at room temperature for 10 minutes. Plasma was then aspirated and placed into plastic tubes with no additives. Plasma was stored at −80 °C until analyzed. For the following 11 days, blood was collected once daily for tT4 determination. Although the time of day that blood was collected was not synchronized, it was always in the morning between 6:00 am and 9:00 am.
Pergolide treatment
Immediately after the 4-hour TRH stimulation test sample was collected, the horses were administered 1 mg pergolide mesylate (Boehringer Ingelheim International GmbH) orally as per the manufacturer’s recommended dose for horses.18 The intact tablet was mixed in a small grain meal, and the horses were observed to ensure each dose was completely consumed. Pergolide was administered for 5 consecutive days at the same dose and time.
Thyrotropin-releasing hormone stimulation test protocol
The protocol described by Sommardahl et al20 was followed. Blood was collected into an evacuated plain tube. Immediately after blood samples were obtained, 1 mL of 1 mg/mL of TRH solution (Wedgewood Pharmacy) was administered IV, and blood was collected 2 and 4 hours later. The harvested serum from all samples was frozen at −80 °C until analysis. The frozen serum collected at the 0- and 4-hour time points was submitted for a complete thyroid panel (tT3, tT4, and fT4). Serum collected at the 2-hour time point was submitted for tT3 determination only as one can expect the maximum increase in tT3 at 2 hours after TRH administration, while tT4 and fT4 are expected to be maximally increased 4 to 6 hours later.1 Although fT3 is the active form of thyroid hormone, this form was not measured as tT4 and fT4 provide a good assessment of the thyroid function of a horse.1
Hormone assay
All frozen serum samples were shipped overnight to the Cornell Animal Health Diagnostic Center Endocrinology Laboratory for analysis. All the assays have been validated in the horse. Free T4 was determined using equilibrium dialysis, and tT4 and tT3 were analyzed by chemiluminescent immunoassay.21
Statistical analysis
Data were assessed for normality using a Shapiro-Wilk test, with normally distributed data presented as mean and SE and nonnormally distributed data presented as median and range. The effects of pergolide administration on tT4 were assessed over time with a 1-way ANOVA, while the effects of pergolide and TRH administration on thyroid function (TRH stimulation tests) were determined using a two-way ANOVA for repeated measures with post hoc comparisons performed when relevant. Effect size and 95% CI are reported. P < .05 was considered significant.
Results
All the horses tolerated the experiments without complications. There was no effect of pergolide administration detected on baseline serum tT4 concentrations during and after treatment (P = .4; Figure 1). Administration of TRH resulted in a significant increase in tT3 (effect size: +165.8 ng/dL [95% CI, 109.4 to 222.2 ng/dL], P < .0001; Figure 2), tT4 (+1.162 µg/dL [95% CI, 0.7135 to 1.610 µg/dL] P < .0001), and fT4 (+1.195 µg/dL [95% CI, 0.7195 to 1.670 µg/dL], P < .0001) concentrations. There was, however, no significant effect of pergolide detected on any thyroid hormone concentration (tT3, P = .2; tT4, P = .7; and fT4, P = .5).
Mean and SD total T4 (tT4) concentrations from 6 horses administered pergolide mesylate for 5 days (gold circles). Black arrows at days 0 and 6 indicate thyrotropin-releasing hormone (TRH) stimulation tests. Black circles indicate days with no pergolide mesylate administration.
Citation: American Journal of Veterinary Research 86, 2; 10.2460/ajvr.24.09.0257
Mean and SD total T3 (tT3; A), total T4 (tT4; B), and free T4 (fT4; C) concentrations from 6 horses before (black circles) and after (gold squares) administration of pergolide mesylate for 5 days. ****P < .0001; ***P < .001; and **P < .01.
Citation: American Journal of Veterinary Research 86, 2; 10.2460/ajvr.24.09.0257
Discussion
This study demonstrated that pergolide administration in horses did not significantly affect serum concentrations of thyroid hormones, including tT3, tT4, and fT4. The administration of TRH, however, led to a significant increase in these hormone concentrations, confirming that the thyroid gland retains its functional capacity, even under pergolide treatment.
Despite the highly protein-bound nature of pergolide, over 90%, which theoretically could displace thyroid hormones from their binding proteins and lower their circulating concentrations, the results of this study do not support this mechanism. This contrasts with other highly protein-bound drugs, such as phenylbutazone, which has been shown to significantly reduce T4 concentrations.11 The absence of such an effect with pergolide suggests that the decrease in thyroid hormone concentrations observed with protein-bound drugs could not be directly or solely attributable to the displacement of bound-thyroid hormones by protein-avid drugs competing for the same binding site.
A proposed mechanism for the effect of phenylbutazone on thyroid hormone concentrations that could extend to other drugs is secondary hypothyroidism.10,11 Secondary hypothyroidism occurs when there is a deficiency in TSH production, often due to dysfunction at the level of the pituitary gland, rather than a primary disorder of the thyroid gland itself.1 To confirm a secondary hypothyroidism, the combination of a TRH stimulation test and a TSH stimulation test would be required. The TRH stimulation test would be consistent with hypothyroidism with no increase in T4 or T3 concentrations, whereas the TSH stimulation test would be normal, indicating a normal thyroid gland function.3,4,22 Since TSH is not accessible in practice and cannot be measured in horses, this hypothesis cannot be explored; however, our data showing a normal response to TRH under pergolide treatment suggest an adequate endogenous secretion of TSH, and what has been observed with phenylbutazone would not apply to pergolide.
In the context of PPID, however, the pituitary gland’s abnormal hormone secretion, specifically the overproduction of pro-opiomelanocortin–related peptides, can disrupt normal TSH regulation, leading to lower circulating thyroid hormones despite an otherwise functional thyroid gland.14 This is an important distinction, as the low thyroid hormone concentrations in PPID horses do not indicate primary thyroid dysfunction but rather a secondary consequence of a primary pituitary disorder. Another explanation proposed by some authors for the role of PPID in thyroid hormone regulation is the nonthyroidal illness syndrome. While this syndrome is recognized in other species, documentation in horses is scarce.12,23 Four mechanisms of nonthyroidal illness syndrome have been described in dogs and people, and it could be assumed they also occur in horses: (1) increased free fraction to bound fraction, (2) decreased conversion of T4 into T3, (3) decreased clearance of reverse T3, and (4) decreased thyrotrope activity (tertiary hypothyroidism).2,24,25 In the absence of a clear definition of this syndrome in horses, further exploration was deemed beyond the scope of this study.
From a clinical perspective, these findings have significant implications for the management of horses with PPID and suggest that low serum thyroid hormone concentrations in PPID horses treated with pergolide are likely due to the disease itself rather than the medication, emphasizing the need for dynamic thyroid function testing before diagnosing hypothyroidism and initiating treatment.
The number of horses recruited was based on a sample size calculation using previously reported data.11 Although a statistical significance could have been reached with a larger sample size, our data suggest a clinical significance is unlikely. The composition of our group, which included 2 horses with ACTH concentration after TRH stimulation consistent with PPID and 4 horses without it, might have limited our ability to detect effects specific to horses with PPID. With only 2 horses in 1 group, relevant comparisons could not be made. Larger groups would have allowed us to decipher the respective roles of PPID and pergolide on thyroid hormone secretion and thyroid gland activity. Nevertheless, the absence of a significant effect of pergolide administration overall would suggest that such a study would have been the repetition of a previous one.14
In conclusion, pergolide administration does not appear to adversely affect thyroid hormone concentrations in horses, and affinity for circulating proteins cannot solely explain the low thyroid hormone concentrations observed with other highly protein-bound drugs. Understanding the underlying mechanisms of thyroid hormone regulation in the context of PPID is crucial for accurate diagnosis and appropriate treatment, preventing unnecessary thyroid hormone supplementation and its potential negative effects on the horse’s health.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org.
Acknowledgments
The authors acknowledge Laura Murray.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
ORCID
F.-R. Bertin https://orcid.org/0000-0002-2820-8431
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