An isolated report describes a reduction in the response to
levothyroxine in a patient also given secobarbital with amobarbital. It seems possible that all barbiturates may interact in this way. Note that
phenobarbital has been shown to reduce the
serum levels of
endogenous thyroid hormones in some studies.
The general importance of this interaction is almost certainly small, but be alert for any evidence of changes in
thyroid status if barbiturates are added or withdrawn from patients taking
levothyroxine.