Warfarin use in Hyperthyroid and Hypothyroid

Hyperthyroidism  

• Increased sensitivity to warfarin
• The mechanism may be that thyroxine increases the affinity of warfarin for receptor sites in the liver, leading to decreased production of clotting factors
• Another mechanism may be increased catabolism of vitamin K dependent clotting factors 

Hypothyroidism

• Decreased response to warfarin
• The mechanism may be decreased catabolism of vitamin K dependent clotting factors

Carbimazole & Propylthiouracil

• Decrease the anticoagulant effect of warfarin by either by increasing its metabolism or indirectly by interfering with absorption

Levothyroxine

• Increase anticoagulant effect 
• Increasing thyroid hormone levels can increase sensitivity to warfarin which may increase the risk of bleeding

REFERENCE :  

1. http://www.medscape.com/viewarticle/410539_3   
2. http://www.aafp.org/afp/2000/1115/p2318.html
3. http://www.inronline.net/index.php/information/179-2/drug-interactions/
4. http://www.rxlist.com/drug-interactions/propylthiouracil-oral-and-warfarin-oral-interaction.htm
5. http://www.rxlist.com/drug-interactions/levothyroxine-oral-and-warfarin-oral-interaction.htm  
6. http://www.bpac.org.nz/BPJ/2007/April/warfarin.aspx
7. LEXICOMP