Open AccessDuration of Effects of Phenylbutazone on Serum Total Thyroxine and Free Thyroxine Concentrations in Horses
Author(s) -
Ramirez Sammy,
Wolfsheimer Karen J.,
Moore Rustin M.,
Mora Francisco,
Bueno AIoisio C.,
Mirza Taskeen
Publication year - 1997
Publication title -
journal of veterinary internal medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.356
H-Index - 103
eISSN - 1939-1676
pISSN - 0891-6640
DOI - 10.1111/j.1939-1676.1997.tb00483.x
Subject(s) - phenylbutazone , medicine , free thyroxine , discontinuation , radioimmunoassay , endocrinology , serum concentration , thyroid function , hormone
The objectives of this study were to determine if phenylbutazone decreased serum thyroxine (TT 4 ) and free thyroxine (FT 4 ) concentrations using 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 discontinued. Serum TT 4 and FT 4 concentrations were determined before and after administration of 4.4 mg/kg of phenylbutazone IV bid for 5 days. Treatment with phenylbutazone caused a significant decrease in TT 4 and FT 4 concentrations ( P < .05). Serum TT 4 concentration significantly decreased after day 4 of treatment and remained significantly below baseline value for 10 days after discontinuing phenylbutazone administration; it returned to a value not different from the baseline value by the 11th day. Serum FT 4 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 returned to a value not different from the baseline value by the 3rd day after discontinuation of phenylbutazone. These results indicate that serum TT 4 and FT 4 should not be used to evaluate thyroid function in horses receiving phenylbutazone. In addition, results should be interpreted cautiously when phenylbutazone has been administered within 2 days (for FT 4 ) or within 10 days (for TT 4 ) of sample collection.