Serum thyroid hormone, insulin, glucose, triglycerides and protein concentrations in normal horses: association with...

The aim of this study was to determine if topical application of dexamethasone affected the serum concentrations of thyroid hormones (triiodothyronine T(3) and thyroxine T(4)), glucose, triglycerides, total protein and insulin in normal horses. Ten horses were treated twice daily for 10 days with 50 g dexamethasone using an ointment formulation. Thyroid hormones and insulin were assayed using standard radioimmunoassay methods, while glucose, triglycerides and total protein were determined using a standard enzymatic method and the Biuret reaction, respectively. An increase in serum glucose and triglyceride concentrations was accompanied by 2-6-fold increases in serum insulin concentrations, but there was no change in serum total protein concentration. Insulin secretion increased with concomitant hyperglycemia and hypertriglyceridemia. A non-significant decline in T(4) secretion was noted. Serum T(3) and T(4) concentrations declined continuously below baseline values from 48 h. Glucose and insulin levels returned to baseline values 3 days after treatment withdrawal, whereas triglycerides reverted to baseline by 7 days. In contrast, baseline values of serum T(3) and T(4) were not reached by 20 days following drug withdrawal. The results indicated that topical administration of dexamethasone affected thyroid function and physiological metabolic functions, which may have implications for potential doping cases in racing horses.