Effects of long-term oral administration of levothyroxine sodium on glucose dynamics in healthy adult horses.
OBJECTIVE
To determine the effects of long-term oral administration of levothyroxine sodium (L-T(4)) on glucose dynamics in adult euthyroid horses.


METHODS
6 healthy adult mares.


METHODS
Horses received L-T(4) (48 mg/d) orally for 48 weeks. Frequently sampled IV glucose tolerance test procedures were performed on 3 occasions (24-hour intervals) before and at 16, 32, and 48 weeks during the treatment period. Data were assessed via minimal model analysis. The repeatability of measurements was evaluated.


RESULTS
During treatment, body weight decreased significantly from the pretreatment value; mean +/- SD weight was 49 +/- 14 kg, 43 +/- 7 kg, and 25 +/- 18 kg less than the pretreatment value at weeks 16, 32, and 48, respectively. Compared with pretreatment findings, 1.8-, 2.4-, and 1.9-fold increases in mean insulin sensitivity (SI) were detected at weeks 16, 32, and 48, respectively; SI was negatively correlated with body weight (r = -0.42; P < 0.001). During treatment, glucose effectiveness increased and the acute insulin response to glucose decreased. Overall mean within-horse coefficients of variation were 5% and 29% for plasma glucose and serum insulin concentrations, respectively, and 33%, 26%, and 23% for SI, glucose effectiveness, and the acute insulin response to glucose, respectively.


CONCLUSIONS
Long-term administration of L-T(4) was associated with weight loss and increased SI in adult euthyroid horses, although other factors may have confounded results. Levothyroxine sodium may be useful for the treatment of obesity and insulin resistance in horses, but further studies are required.
Frank
Nicholas
N
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996, USA.
Elliott
Sarah B
SB
Boston
Raymond C
RC
D000284:Administration, Oral; D000818:Animals; D001786:Blood Glucose / Q000187:drug effects*; D004334:Drug Administration Schedule; D005260:Female; D006262:Health; D006736:Horses / Q000502:physiology*; D007328:Insulin / Q000378:metabolism; D013974:Thyroxine / Q000008:administration & dosage* / Q000097:blood / Q000494:pharmacology*; D013997:Time Factors; D015431:Weight Loss / Q000187:drug effects
D001786:Blood Glucose; D007328:Insulin; D013974:Thyroxine