Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares.
Abstract: To determine the effects of levothyroxine sodium (L-T4) on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone (TRH) in euthyroid horses. Methods: 12 healthy adult mares. Methods: 8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provided for 2 weeks. Four additional horses remained untreated. Serum concentrations of total triiodothyronine (tT3), total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) were measured in samples obtained at weeks 0, 2, 4, 6, and 8; 1.2 mg of TRH was then administered i.v., and serum concentrations of thyroid gland hormones were measured 2 and 4 hours after injection. Serum reverseT3 (rT3) concentration was also measured in the samples collected at weeks 0 and 8. Results: Treated horses lost a significant amount of weight (median, 19 kg). Significant treatment-by-time effects were detected for serum tT3, tT4, fT3, fT4, and TSH concentrations, and serum tT4 concentrations were positively correlated (r, 0.95) with time (and therefore dosage) in treated horses. Mean +/- SD serum rT3 concentration significantly increased in treated horses (3.06 +/- 0.51 nmol/L for week 8 vs 0.74 +/- 0.22 nmol/L for week 0). Serum tT3, tT4, fT3, and TSH concentrations in response to TRH injections differed significantly between treated and untreated horses. Conclusions: Administration of levothyroxine sodium increased serum tT4 concentrations and blunted responses toTRH injection in healthy euthyroid horses.
Publication Date: 2005-07-13 PubMed ID: 16008227DOI: 10.2460/ajvr.2005.66.1025Google Scholar: Lookup
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- Comparative Study
- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The study investigates the impacts of levothyroxine sodium on various thyroid hormones and their response to thyrotropin-releasing hormone injection in horses. The findings indicate that levothyroxine sodium elevates certain thyroid hormone concentrations and affects the response of these hormones to hormone injections.
Research Methodology
- The research involved 12 healthy adult mares, with 8 receiving varying dosages of Levothyroxine Sodium (L-T4) over eight weeks and 4 left untreated for control comparison.
- The treated horses received increasing amounts of L-T4 for the duration of the study. Each increment dosage, varying between 24mg to 96mg, was provided for two weeks.
- For both groups of horses, their serum was sampled at intervals (weeks 0, 2, 4, 6, and 8). These samples were tested for concentrations of various hormones such as total triiodothyronine (tT3), total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH).
- Additionally, 1.2mg of Thyrotropin-Releasing Hormone (TRH) was administered intravenously to the horses and then their thyroid hormone concentrations measured again two and four hours post-injection.
- The researchers also assessed the serum concentration of reverse Triiodothyronine (rT3) at the beginning and end of the study (weeks 0 and 8).
Key Findings
- The results showed that the horses receiving Levothyroxine Sodium lost a significant amount of weight, with a median loss of 19kg.
- Significant variations were identified in serum concentrations over time for tT3, tT4, fT3, fT4, and TSH. In the treated horses, serum tT4 concentrations increased with time and dosage.
- The study also recorded a significant increase in serum rT3 concentration in the treated horses at the end of the study compared to the beginning.
- Post TRH injection, significant differences in serum tT3, tT4, fT3, and TSH concentration responses were observed between the treated and untreated horses.
Conclusion
- In conclusion, the administration of Levothyroxine Sodium resulted in an increase in serum tT4 concentration, impacted other hormones as well and affected the body’s response to TRH injection in healthy euthyroid horses.
Cite This Article
APA
Sommardahl CS, Frank N, Elliott SB, Webb LL, Refsal KR, Denhart JW, Thompson DL.
(2005).
Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares.
Am J Vet Res, 66(6), 1025-1031.
https://doi.org/10.2460/ajvr.2005.66.1025 Publication
Researcher Affiliations
- Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996, USA.
MeSH Terms
- Analysis of Variance
- Animals
- Body Weight / drug effects
- Dose-Response Relationship, Drug
- Female
- Horses / metabolism
- Thyroid Gland / drug effects
- Thyroid Hormones / blood
- Thyrotropin-Releasing Hormone / pharmacology
- Thyroxine / pharmacology
- Time Factors
Citations
This article has been cited 2 times.- Breuhaus BA. Thyroid function and dysfunction in term and premature equine neonates.. J Vet Intern Med 2014 Jul-Aug;28(4):1301-9.
- Hilderbran AC, Breuhaus BA, Refsal KR. Nonthyroidal illness syndrome in adult horses.. J Vet Intern Med 2014 Mar-Apr;28(2):609-17.
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