Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin-Stimulating Hormone in Horses with PPID Compared with Age-Matched Normal Horses.
Abstract: Glucocorticoids are known to exert inhibitory action on the hypothalamic-pituitary-thyroid axis. With recent evidence that free plasma cortisol and urinary excretion of cortisol metabolites may be increased in horses with pituitary pars intermedia dysfunction (PPID), it is important to further examine thyroid function in horses with PPID. To test the hypothesis that serum thyrotropin (TSH) concentrations are decreased in horses with PPID, baseline serum thyroid hormone and TSH concentrations, and responses to TSH-releasing hormone (TRH), were compared between 12 horses diagnosed as having PPID and 14 age-matched normal horses. Horses with PPID had resting serum concentrations of free thyroxine by equilibrium dialysis (fT4D) and TSH that were significantly lower than serum concentrations of fT4D and TSH in age-matched normal horses. Serum concentrations of total T4 and total and free triiodothyronine (T3) were also lower in horses with PPID compared with normal horses, but the differences did not reach statistical significance. Thyroid hormone and TSH responses to TRH administration were not different between horses with PPID and normal horses. In conclusion, serum fT4D concentrations are decreased in horses with PPID without an appropriate increase in serum TSH concentrations. Normal serum thyroid hormone and TSH concentration responses to exogenous TRH administration support the theory that increased glucocorticoid activity in horses with PPID may exert prolonged tonic suppression, but not complete inhibition, of TRH and subsequent TSH release, similar to what has been observed in other species.
Copyright © 2019 Elsevier Inc. All rights reserved.
Publication Date: 2019-01-30 PubMed ID: 31002090DOI: 10.1016/j.jevs.2019.01.008Google Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The research focuses on the thyroid function in horses with pituitary pars intermedia dysfunction (PPID), suggesting that free thyroxine and thyrotropin levels are significantly lower in these horses as compared to normal ones.
Background of the Study
- This study is built upon the knowledge that glucocorticoids are known to exert inhibitory action on the hypothalamic-pituitary-thyroid axis.
- Recent evidence shows that free plasma cortisol and urinary excretion of cortisol metabolites may be increased in horses with pituitary pars intermedia dysfunction (PPID).
- This raised the need to further examine thyroid function in horses with PPID.
Objective and Methodology
- The objective of the study was to ascertain whether serum thyrotropin (TSH) concentrations are decreased in horses with PPID.
- The baseline serum thyroid hormone and TSH concentrations, along with responses to TSH-releasing hormone (TRH), were analysed and compared between 12 horses diagnosed as having PPID and 14 age-matched normal horses.
Findings
- The study found that horses with PPID had resting serum concentrations of free thyroxine by equilibrium dialysis (fT4D) and TSH that were significantly lower than those in age-matched normal horses.
- It was also observed that serum concentrations of total T4 and total and free triiodothyronine (T3) were also lower in horses with PPID compared with normal horses, but these differences were not statistically significant.
- No difference was found in thyroid hormone and TSH responses to TRH administration between horses with PPID and normal horses.
Conclusion
- The study concludes that in horses with PPID, serum fT4D concentrations are reduced, without an associated increase in serum TSH concentrations.
- Despite this, normal serum thyroid hormone and TSH concentration responses to the administration of TRH are observed.
- This supports the theory that increased glucocorticoid activity in horses with PPID may exert a prolonged tonic suppression (but not complete inhibition) of TRH and the subsequent release of TSH, mirroring what has been observed in other species.
Cite This Article
APA
Breuhaus BA.
(2019).
Thyroid Hormone and Thyrotropin Concentrations and Responses to Thyrotropin-Stimulating Hormone in Horses with PPID Compared with Age-Matched Normal Horses.
J Equine Vet Sci, 75, 35-40.
https://doi.org/10.1016/j.jevs.2019.01.008 Publication
Researcher Affiliations
- Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, 1060 William Moore Drive, Raleigh, NC. Electronic address: betta_breuhaus@ncsu.edu.
MeSH Terms
- Animals
- Equidae
- Horses
- Pituitary Diseases / veterinary
- Thyroid Hormones
- Thyrotropin
- Thyrotropin-Releasing Hormone
Citations
This article has been cited 2 times.- Kirkwood NC, Hughes KJ, Stewart AJ. Pituitary Pars Intermedia Dysfunction (PPID) in Horses. Vet Sci 2022 Oct 10;9(10).
- Ferlazzo A, Cravana C, Fazio E, Medica P. The different hormonal system during exercise stress coping in horses. Vet World 2020 May;13(5):847-859.
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