DURATION OF EFFECTS OF PHENYLBUTAZONE ON SERUM TOTAL THYROXINE AND FREE-THYROXINE CONCENTRATIONS IN HORSES

The objectives of this study were to determine if phenylbutazone decre ased serum thyroxine (TT4) and free thyroxine (FT4) concentrations usi ng radioimmunoassay and equilibrium dialysis techniques in horses, and , if so, an additional objective was to determine the duration of this decreased concentration once phenylbutazone administration was discon tinued. Serum TT4 and FT4 concentrations were determined before and af ter administration of 4.4 mg/kg of phenylbutazone IV bid for 5 days. T reatment with phenylbutazone caused a significant decrease in TT4 and FT4 concentrations (P < .05), Serum TT4 concentration significantly de creased after day 4 of treatment and remained significantly below base line value for 10 days after discontinuing phenylbutazone administrati on; it returned to a value not different from the baseline value by th e 11th day. Serum FT4 concentration significantly decreased after day 4 of treatment and remained significantly below the baseline value for only 1 day after phenylbutazone administration was discontinued; it r eturned to a value not different from the baseline value by the 3rd da y after discontinuation of phenylbutazone. These results indicate that serum TT4 and FT4 should not be used to evaluate thyroid function in horses receiving phenylbutazone. In addition, results should be interp reted cautiously when phenylbutazone has been administered within 2 da ys (for FT4) or within 10 days (for TT4) of sample collection. Copyrig ht (C) 1997 by the American College of Veterinary Infernal Medicine.