To investigate the effect of high doses of orally administered levothyroxine sodium (LT4) on serum concentrations of triiodothyronine (T3) and thyroxine (T4) in euthyroid horses.
12 healthy adult horses.
10 horses initially received water (vehicle) or 240 mg (5X treatment) or 480 mg (10× treatment) of LT4, and blood samples were collected at baseline (0 hours) and 0.5, 1, 2, 4, 6, 8, 10, 12, 18, 24, 48, 72, 96, and 120 hours after treatment to measure serum T3 and T4 concentrations. Three horses then received 480 mg of LT4 for 14 days, and T4 concentration was measured on days 0, 14, 21, 28, and 35. Changes in T3 and T4 concentrations were compared over time and among treatments.
One-time administration of LT4 resulted in variable but significant increases in both T3 and T4 concentrations for up to 120 hours; however, T3 and T4 concentrations rarely exceeded reference intervals with either treatment. Prolonged administration of 480 mg of LT4 resulted in a 15-fold increase in T4 concentration after 14 days, but concentration returned to day 0 values within 21 days after LT4 administration was discontinued.
CONCLUSIONS AND CLINICAL RELEVANCE
In euthyroid horses, administration of a high dose of LT4 resulted in mild increases in thyroid hormone concentrations; however, prolonged administration of high doses of LT4 resulted in markedly increased thyroid hormone concentrations that returned to pretreatment values within 3 weeks after discontinuation of LT4 administration. These results indicated complex kinetics of LT4 and suggested a possible saturation of T4 excretion in euthyroid horses.
Objective—To evaluate the effect of ovariectomy on insulin sensitivity in horses and determine whether the effects of suppression of the hypothalamo-pituitary-adrenal axis differ before and after ovariectomy.
Animals—6 healthy mares.
Procedures—The horses underwent an IV glucose tolerance test (IVGTT), an insulin sensitivity test, and a dexamethasone suppression test before and 5 weeks after ovariectomy. Body weight, serum cortisol and plasma ACTH concentrations, serum insulin-to-blood glucose concentration ratios, and changes in blood glucose concentration with time after injection of glucose or insulin were compared before and after ovariectomy.
Results—The dexamethasone injection resulted in a decrease in serum cortisol concentration before and after ovariectomy. In all horses, baseline plasma ACTH concentrations were within the reference range before and after ovariectomy. For each mare, results of an IVGTT before and after ovariectomy were considered normal. No significant differences in basal blood glucose concentration or time to reach baseline glucose concentration after an IVGTT were observed. Basal serum insulin concentration and serum insulin-to-blood glucose concentration ratios were not significantly different before or after ovariectomy, nor was the mean time to attain a 50% decrease in blood glucose concentration after insulin injection.
Conclusions and Clinical Relevance—Results indicated that ovariectomy does not appear to modify dexamethasone response in horses and that it does not modify short-term measures of insulin sensitivity. Findings suggested that horses undergoing ovariectomy are not at higher risk of developing equine metabolic syndrome or hypothalamo-pituitary-adrenal axis dysfunction and associated morbidity.
Objective—To develop and assess the short-term feasibility, maintenance, and complications associated with percutaneous endoscopic gastrostomy (PEG) tube placement in standing horses.
Animals—6 adult horses.
Procedures—Feasibility of the technique was evaluated in 2 horses. In each of 4 other horses, a PEG tube was maintained for 14 days and used to provide fluid requirements during the latter 7 days, before removal. Following air inflation of the stomach, each PEG tube was placed via a left intercostal approach; proper tube location was ascertained by percutaneous ultrasonography and gastroscopy. The horses underwent physical examinations, CBCs, and peritoneal fluid analyses before and at intervals after tube placement. Seven days after tube removal, horses were euthanized and necropsied.
Results—Placement of a PEG tube was feasible in all 6 horses. The 4 horses assessed long term tolerated water administration through the PEG tube and remained clinically stable throughout the 21-day experiment. However, during the period PEG tubes were in place, significant increases in some peritoneal and hematologic variables were detected. Postmortem evaluation revealed localized peritonitis in 1 horse and body wall inflammation along the PEG tube tracks in 3 additional horses.
Conclusions and Clinical Relevance—Placement and maintenance of a PEG tube were tolerated well by the study horses, although peritoneal and systemic inflammation were detectable. Fluid requirements were adequately met with this technique, which could provide an alternative method for managing chronically dysphagic horses. Nevertheless, further research is warranted to evaluate the feasibility of enteral feeding by use of this approach in horses.