Effect of phenytoin therapy on thyroid function.

Serum total and free fraction of thyroxine and triiodothyronine and urinary losses of unconjugated hormones in normal subjects and in patients treated long-term with therapeutic doses of phenytoin have been measured. Decreases in serum total hormone concentrations with increased free fractions and resultant significant increase in the concentration of free thyroxine but not triiodothyronine were apparent in phenytoin-treated subjects. However, serum free hormone concentrations remained within the euthyroid range. These changes in serum free hormone concentration were reflected by an increased urinary loss of unconjugated thyroxine, but normal excretion of unconjugated triiodothyronine. Phenytoin in therapeutic doses displaces thyroxine and to a lesser extent, triiodothyronine from binding proteins in serum and thus increases peripheral clearance of thyroid hormones.