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  Discussion on Thyroid supplementation for Metabolic SynDrOme?
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DrO

Posted on Monday, Aug 22, 2005 - 10:26 am:

Long time readers of Horseadvice know that I have been on a crusade against the diagnosis and treatment of "hypothyroidism" in adult horses. Research was clear for the past 20 years: it was exceedingly rare in adults and the simply T3/T4 test did not accurately reflect what was happening with the horses thyroids. There was a dearth of information on the safety or this practice.

This is all true however two recent papers show the good and bad of this practice. Thyroid supplementation in very easy keepers, particularly those that suffer from Metabolic SynDrOme increases insulin sensitivity and has resulted in weight loss. However the second paper shows it does disrupt normal thyroid action.

Certainly the thousands of horses this has seen thyroid hormone supplementation therapy seem not to have trouble with the therapy, but I believe that further work should be done before recommendations can be made.
DrO

Am J Vet Res. 2005 Jun;66(6):
Effects of oral administration of levothyroxine sodium on concentrations of plasma lipids, concentration and composition of very-low-density lipoproteins, and glucose dynamics in healthy adult mares.

Frank N, Sommardahl CS, Eiler H, Webb LL, Denhart JW, Boston RC.

Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996, USA.

OBJECTIVE: To evaluate glucose and lipid metabolism in healthy adult horses administered levothyroxine sodium (L-T4). ANIMALS: 12 healthy adult mares. PROCEDURE: 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 provide between 7 AM and 8 AM in the morning grain meal for 2 weeks. Four additional horses remained untreated. Serum concentrations of nonesterified fatty acids, triglyceride (TG), total cholesterol (TC), and very-low-density lipoprotein (VLDL) were measured and composition of VLDL examined in samples obtained between 8 AM and 9 AM at weeks 0, 2, 4, 6, and 8. Glucose dynamics were assessed by use of a combined IV glucose-insulin tolerance test (IVGITT) conducted before and at the end of the 8-week treatment period. Data for each combined IVGITT were interpreted by use of the minimal model. RESULTS: Plasma TG, TC, and VLDL concentrations significantly decreased over time in treated horses. At the completion of the 8-week treatment period, mean plasma VLDL concentration was 46% of the mean value for week 0 in treated horses. Insulin sensitivity significantly increased (> 2-fold) in treated horses, but glucose effectiveness and net insulin response were not affected. Levothyroxine sodium significantly increased the rate of insulin disposal. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of L-T4 decreases blood lipid concentrations, improves insulin sensitivity, and increases insulin disposal in horses. Levothyroxine sodium may have potential as a treatment for horses with reduced insulin sensitivity.

Am J Vet Res. 2005 Jun;66(6):1025-31.
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.
Sommardahl CS, Frank N, Elliott SB, Webb LL, Refsal KR, Denhart JW, Thompson DL Jr.
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996, USA.

OBJECTIVE: 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. ANIMALS: 12 healthy adult mares. PROCEDURE: 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 AND CLINICAL RELEVANCE: Administration of levothyroxine sodium increased serum tT4 concentrations and blunted responses toTRH injection in healthy euthyroid horses.
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