To our knowledge, raloxifene hydrochloride, a selective estrogen receptor 
modulator, has never been reported to interfere with absorption of 
levothyroxine. We describe a 79-year-old woman with chronic, treated primary 
hypothyroidism, presenting with increasing levothyroxine requirement while 
taking raloxifene at the same time as levothyroxine. For two 6- to 8-week 
periods, we separated the ingestion of raloxifene and levothyroxine by about 12 
hours. In addition, we tested the absorption of 1.0 mg of levothyroxine sodium 
with and without the coadministration of 60 mg of raloxifene hydrochloride on 2 
separate occasions by collecting serial blood samples for 6 hours. 
Hypothyroidism occurred in a reproducible fashion whenever levothyroxine and 
raloxifene were administered together and improved whenever they were taken 
separately. Combined administration of levothyroxine and raloxifene resulted in 
lower levels of serum thyroxine compared with administration of levothyroxine 
alone. By a yet unknown mechanism, raloxifene caused malabsorption of 
levothyroxine in our patient when coadministered.