• 1.

    Sommardahl CS, Frank N, Elliott SB, et al. 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 2005;66:10251031.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2.

    Frank N, Sommardahl CS, Eiler H, et al. Effects of oral administration of levothyroxine sodium on concentrations of plasma lipids, concentration and composition of very-lowdensity lipoproteins, and glucose dynamics in healthy adult mares. Am J Vet Res 2005;66:10321038.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Biondi B, Fazio S, Carella C, et al. Cardiac effects of long term thyrotropin-suppressive therapy with levothyroxine. J Clin Endocrinol Metab 1993;77:334338.

    • Search Google Scholar
    • Export Citation
  • 4.

    Ching GW, Franklyn JA, Stallard TJ, et al. Cardiac hypertrophy as a result of long-term thyroxine therapy and thyrotoxicosis. Heart 1996;75:363368.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 5.

    Shapiro LE, Sievert R, Ong L, et al. Minimal cardiac effects in asymptomatic athyreotic patients chronically treated with thyrotropin-suppressive doses of L-thyroxine. J Clin Endocrinol Metab 1997;82:25922595.

    • Search Google Scholar
    • Export Citation
  • 6.

    Sawin CT, Geller A, Wolf PA, et al. Low serum thyrotropin concentrations as a risk factor for atrial fibrillation in older persons. N Engl J Med 1994;331:12491252.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7.

    Casu M, Cappi C, Patrone V, et al. Sympatho-vagal control of heart rate variability in patients treated with suppressive doses of L-thyroxine for thyroid cancer. Eur J Endocrinol 2005;152:819824.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8.

    Frank N, Elliott SB, Boston RC, et al. Effects of long-term oral administration of levothyroxine sodium on glucose dynamics in healthy adult horses. Am J Vet Res 2008;69:7681.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Sojka JE, Johnson MA, Bottoms GD. Serum triiodothyronine, total thyroxine, and free thyroxine concentrations in horses. Am J Vet Res 1993;54:5255.

    • Search Google Scholar
    • Export Citation
  • 10.

    Ramirez S, Wolfsheimer KJ, Moore RM, et al. Duration of effects of phenylbutazone on serum total thyroxine and free thyroxine concentrations in horses. J Vet Intern Med 1997;11:371374.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11.

    Peek SF, Marques FD, Morgan J, et al. Atypical acute monensin toxicosis and delayed cardiomyopathy in belgian draft horses. J Vet Intern Med 2004;18:761764.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12.

    Long KJ, Bonagura JD, Darke PG. Standardised imaging technique for guided M-mode and Doppler echocardiography in the horse. Equine Vet J 1992;24:226235.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13.

    Patteson MW, Gibbs C, Wotton PR, et al. Echocardiographic measurements of cardiac dimensions and indices of cardiac function in normal adult thoroughbred horses. Equine Vet J Suppl 1995;19:1827.

    • Search Google Scholar
    • Export Citation
  • 14.

    Buhl R, Ersboll AK, Eriksen L, et al. Sources and magnitude of variation of echocardiographic measurements in normal standardbred horses. Vet Radiol Ultrasound 2004;45:505512.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15.

    Marr CM, Reef VB, Reimer JM, et al. An echocardiographic study of atrial fibrillation in horses: before and after conversion to sinus rhythm. J Vet Intern Med 1995;9:336340.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16.

    Slater JD, Herrtage ME. Echocardiographic measurements of cardiac dimensions in normal ponies and horses. Equine Vet J Suppl 1995;19:2832.

    • Search Google Scholar
    • Export Citation
  • 17.

    Alberts MK, McCann JP, Woods PR. Hemithyroidectomy in a horse with confirmed hyperthyroidism. J Am Vet Med Assoc 2000;217:10511054.

  • 18.

    Ramirez S, McClure JJ, Moore RM, et al. Hyperthyroidism associated with a thyroid adenocarcinoma in a 21-year-old gelding. J Vet Intern Med 1998;12:475477.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 19.

    Slack JA, McGuirk SM, Erb HN, et al. Biochemical markers of cardiac injury in normal, surviving septic, or nonsurviving septic neonatal foals. J Vet Intern Med 2005;19:577580.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20.

    Sticker LS, Thompson DL Jr, Bunting LD, et al. Dietary protein and (or) energy restriction in mares: plasma glucose, insulin, nonesterified fatty acid, and urea nitrogen responses to feeding, glucose, and epinephrine. J Anim Sci 1995;73:136144.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21.

    Sojka JE. Hypothyroidism in horses. Compend Contin Educ Pract Vet 1995;17:845852.

  • 22.

    Sojka JE, Levy M. Evaluation of endocrine function. Vet Clin North Am Equine Pract 1995;11:415435.

  • 23.

    Rothschild CM, Hines MT, Breuhaus B, et al. Effects of trimethoprim-sulfadiazine on thyroid function of horses. J Vet Intern Med 2004;18:370373.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 24.

    Meredith TB, Dobrinski I. Thyroid function and pregnancy status in broodmares. J Am Vet Med Assoc 2004;224:892894.

  • 25.

    Breuhaus BA. Thyroid function in mature horses ingesting endophyte-infected fescue seed. J Am Vet Med Assoc 2003;223:340345.

  • 26.

    Toribio RE, Duckett WM. Thyroid gland. In: Reed SM, Bayly WM, Sellon DC, eds. Equine internal medicine. 2nd ed. Philadelphia: WB Saunders Co, 2004;13401356.

    • Search Google Scholar
    • Export Citation
  • 27.

    Eisenberg M, Samuels M, DiStefano JJ III. L-T4 bioequivalence and hormone replacement studies via feedback control simulations. Thyroid 2006;16:12791292.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 28.

    Roos A, Linn-Rasker SP, van Domburg RT, et al. The starting dose of levothyroxine in primary hypothyroidism treatment: a prospective, randomized, double-blind trial. Arch Intern Med 2005;165:17141720.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 29.

    Bauer M, London ED, Rasgon N, et al. Supraphysiological doses of levothyroxine alter regional cerebral metabolism and improve mood in bipolar depression. Mol Psychiatry 2005;10:456469.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 30.

    Mercuro G, Panzuto MG, Bina A, et al. Cardiac function, physical exercise capacity, and quality of life during long-term thyrotropinsuppressive therapy with levothyroxine: effect of individual dose tailoring. J Clin Endocrinol Metab 2000;85:159164.

    • Crossref
    • Search Google Scholar
    • Export Citation

Advertisement

Effects of long-term oral administration of levothyroxine sodium on serum thyroid hormone concentrations, clinicopathologic variables, and echocardiographic measurements in healthy adult horses

Nicholas FrankDepartment of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

Search for other papers by Nicholas Frank in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
Benjamin R. BuchananDepartment of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

Search for other papers by Benjamin R. Buchanan in
Current site
Google Scholar
PubMed
Close
 DVM
, and
Sarah B. ElliottDepartment of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

Search for other papers by Sarah B. Elliott in
Current site
Google Scholar
PubMed
Close
 BS

Abstract

Objective—To determine the effects of long-term oral levothyroxine sodium (L-T4) administration on serum thyroid hormone concentrations, thyroid gland function, clinicopathologic variables, and echocardiographic examination measurements in adult euthyroid horses.

Animals—6 healthy adult mares.

Procedures—Horses received L-T4 (48 mg/d) orally for 48 weeks. Every 4 weeks, physical examinations were performed; blood samples were collected for CBC, plasma biochemical analyses, and assessments of serum total triiodothyronine (tT3) and thyroxine (tT4) concentrations. Plasma creatine kinase MB activity and cardiac troponin I concentration were also measured. Echocardiographic examinations were performed before and at 16, 32, and 48 weeks during the treatment period.

Results—During the treatment period, mean body weight decreased significantly; heart rate varied significantly, but the pattern of variation was not consistent. Significant time effects were detected for certain clinicopathologic variables, but mean values remained within reference ranges. Cardiac troponin I was only detectable in 8 of 24 plasma samples (concentration range, 0.01 to 0.03 ng/mL). Serum creatine kinase MB activity did not change significantly over time. Compared with the pretreatment value, 5.4-, 4.0-, and 3.7-fold increases in mean serum tT4 concentrations were detected at 16, 32, and 48 weeks, respectively. Some cardiac measurements changed significantly over time, but mean values remained within published reference ranges. Mean fractional shortening was lower than the pretreatment mean value at 16 and 32 weeks.

Conclusions and Clinical Relevance—In horses, long-term oral administration of 48 mg of L-T4/d significantly increased serum tT4 concentrations and did not appear to adversely affect health.

Abstract

Objective—To determine the effects of long-term oral levothyroxine sodium (L-T4) administration on serum thyroid hormone concentrations, thyroid gland function, clinicopathologic variables, and echocardiographic examination measurements in adult euthyroid horses.

Animals—6 healthy adult mares.

Procedures—Horses received L-T4 (48 mg/d) orally for 48 weeks. Every 4 weeks, physical examinations were performed; blood samples were collected for CBC, plasma biochemical analyses, and assessments of serum total triiodothyronine (tT3) and thyroxine (tT4) concentrations. Plasma creatine kinase MB activity and cardiac troponin I concentration were also measured. Echocardiographic examinations were performed before and at 16, 32, and 48 weeks during the treatment period.

Results—During the treatment period, mean body weight decreased significantly; heart rate varied significantly, but the pattern of variation was not consistent. Significant time effects were detected for certain clinicopathologic variables, but mean values remained within reference ranges. Cardiac troponin I was only detectable in 8 of 24 plasma samples (concentration range, 0.01 to 0.03 ng/mL). Serum creatine kinase MB activity did not change significantly over time. Compared with the pretreatment value, 5.4-, 4.0-, and 3.7-fold increases in mean serum tT4 concentrations were detected at 16, 32, and 48 weeks, respectively. Some cardiac measurements changed significantly over time, but mean values remained within published reference ranges. Mean fractional shortening was lower than the pretreatment mean value at 16 and 32 weeks.

Conclusions and Clinical Relevance—In horses, long-term oral administration of 48 mg of L-T4/d significantly increased serum tT4 concentrations and did not appear to adversely affect health.

Contributor Notes

Supported by a grant from Lloyd Incorporated, Shenandoah, Iowa.

Address correspondence to Dr. Frank.