title: Raloxifene causing malabsorption of levothyroxine.
abstract: 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.
authors.affiliation: Department of Endocrinology, Diabetes & Metabolism, Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA.
authors.forename: Elias S
authors.initials: ES
authors.lastname: Siraj
authors.forename: Manjula K
authors.initials: MK
authors.lastname: Gupta
authors.forename: S Sethu K
authors.initials: SS
authors.lastname: Reddy
mesh_terms: D000042:Absorption; D000368:Aged; D004347:Drug Interactions; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000097:blood / Q000188:drug therapy*; D020849:Raloxifene Hydrochloride / Q000494:pharmacology*; D020845:Selective Estrogen Receptor Modulators / Q000494:pharmacology*; D013974:Thyroxine / Q000097:blood / Q000493:pharmacokinetics* / Q000627:therapeutic use
chemical_list: D020845:Selective Estrogen Receptor Modulators; D020849:Raloxifene Hydrochloride; D013974:Thyroxine