title: Exacerbation of warfarin-induced anticoagulation by hyperthyroidism.
abstract: OBJECTIVE
  To alert clinicians to the potential interaction between warfarin and hyperthyroidism.
  
  
  METHODS
  We present two case reports and compare findings with those in a control population of patients with hyperthyroidism who were not receiving anticoagulant therapy.
  
  
  RESULTS
  In two patients, severe coagulopathy was precipitated by the interaction of warfarin and increased thyroid hormone levels. In both cases, the patients also demonstrated resistance to vitamin K therapy, which persisted for several days. We also studied five control patients with hyperthyroidism who were not taking anticoagulant drugs; no effect of thyroid hormone on the plasma levels of vitamin K-dependent clotting factors was noted. One of our patients with hyperthyroidism who was taking warfarin had levels of warfarin in the serum that were 5 times the therapeutic range; this finding suggests that the protein binding or absorption of warfarin may be altered in such patients.
  
  
  CONCLUSIONS
  Multiple factors may contribute to the enhanced effect of warfarin seen in patients with hyperthyroidism, including altered metabolism of vitamin K-dependent clotting factors, altered metabolism of warfarin, or decreased protein binding of the drug. Patients with hyperthyroidism should be given lower doses of warfarin to avoid severe coagulopathy.
authors.affiliation: Division of Hematology/Oncology, National Naval Medical Center, Bethesda, Maryland 20889-5600, USA.
authors.forename: J P
authors.initials: JP
authors.lastname: Chute
authors.forename: C P
authors.initials: CP
authors.lastname: Ryan
authors.forename: G
authors.initials: G
authors.lastname: Sladek
authors.forename: K M
authors.initials: KM
authors.lastname: Shakir