There are conflicting statements in the literature on the effect of phenytoin (diphenylhydantoin; Dilantin) on certain thyroid tests. Dilantin is known to compete for binding sites on TBG, but it may also affect thyroid tests due to activation (“induction”) of liver microsomal enzymes, resulting in accelerated metabolic alteration of T4. The majority of reports indicate that T4, free T4, and T3-RIA values are somewhat decreased from pretreatment levels. The T4 level is said to decrease about 25%-33% from pretreatment level, whereas the T3-RIA value decreases about 15%. In one study, the T4 level decreased below reference range in 40% of patients, whereas the T3-RIA value remained within reference range in all patients. The THBR, TSH and TRH tests are not significantly affected.