An isolated report describes a patient, previously well maintained on levothyroxine, who developed
clinical hypothyroidism when also given
phenytoin.
Phenytoin can
reduce
endogenous serum thyroid hormone levels, but
clinical hypothyroidism caused by an interaction seems to be rare.
The general importance of this interaction seems small, but be alert for any evidence of changes in
thyroid status if
phenytoin is added or withdrawn from patients taking
levothyroxine.