Effect of 5,5'-diphenylhydantoin on the metabolism of L-thyroxine-131-I in the rat.

Abstract

Because 5,5'-diphenylhydantoin (DPH) decreases the concentration of serum protein-bound iodine (PBI) in man, we have studied the effect of this drug on the metabolism of L-thyroxine in the rat and in the isolated rat liver. DPH produced a decrease in concentration of PBI in the rat although it did not alter the uptake of T3-l31I from plasma by erythrocytes. DPH was administered to rats in daily doses of 25 mg for 3 and 13 days, the bile duct was then cannulated, and L-T4 was administered by intravenous injection. DPH produced an increased output of 131I in the bile of these rats, and the major 13lI compound in bile was T4 glucuronide. Less radioactive iodide from T4 was excreted in the urine by these rats. DPH altered the metabolism of L-T4 in the isolated perfused rat liver. When DPH (25 mg) was added directly to blood perfusing the rat liver, the rate of disappearance of T4 from the blood was delayed somewhat, but the rate of accumulation of radioactive iodide in the blood was unaltered. The uptake of T...