Duration of effects of phenylbutazone on serum total thyroxine and free thyroxine concentrations in horses.

This research studied the impact of the drug phenylbutazone on the levels of certain hormones in horses. It was found that the medication significantly decreased serum thyroxine levels, and these effects persisted for several days even after discontinuing the drug.

Overview of Study and Objectives

• The main focus of this research was to investigate if the drug phenylbutazone alters the concentrations of the hormones serum thyroxine (TT4) and free thyroxine (FT4) in horses.
• The researchers measured these hormone levels both before and after administering the drug to the animals, using established methods such as radioimmunoassay and equilibrium dialysis techniques.
• Another objective was to understand how long the decreased hormone concentrations caused by the drug would persist after discontinuing its administration.

Methodology and Results

• The horses were given an intravenous dosage of 4.4 mg/kg of phenylbutazone twice a day for five days.
• The study discovered that treatment with phenylbutazone indeed caused a significant drop in both TT4 and FT4 concentrations, with these effects deemed significant if the probability of them occurring by chance was less than 5% (P < .05).
• The concentration of TT4 decreased significantly after the fourth day of treatment and remained below the baseline value (the level before drug administration) for ten days after the drug was stopped. By the 11th day, it had returned to the baseline value.
• Moreover, the FT4 concentration was found to have significantly dropped after the fourth day of treatment as well. However, it returned to the baseline value by the third day after stopping drug administration, suggesting that FT4 levels recover more quickly than TT4 levels.

Implications of the Study

• These findings indicate that using serum TT4 and FT4 to evaluate thyroid function in horses on a phenylbutazone regimen may not yield accurate information, due to the drug’s effect on these hormone levels.
• Moreover, these hormone levels should be interpreted mindfully if the horse has been given phenylbutazone within two days of sample collection for FT4, or within ten days for TT4, as the drug could still be influencing these concentrations.