4cfc52b1__for_Professionals___Drugs_com__Interaction
[Drug or Test] Test, protein-bound iodine (PBI) [Comments] The 24-hour I 131 thyroidal uptake is apparently not affected Phenytoin may produce increased resin or red cell T3 uptake valuesb Lowered PBI values do not occur unless phenytoin is administered for 1 week or longer, and altered values persist for 7–10 days after phenytoin is discontinuedb [Interaction] 
Patients receiving phenytoin have shown reduced protein-bound iodine (PBI) test values without lowered triiodothyronine (T3) values and without clinical symptoms of hypothyroidism; free thyroxine concentrations may also be decreasedb
Phenytoin Sodium Monograph for Professionals - Drugs.com
Interaction