levothyroxine sodium × warfarin

Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine is initiated or its dose is raised in a patient becoming euthyroid, warfarin effect increases – INR rises and the warfarin dose must be reduced. Conversely, hypothyroidism or a tyroxine dose reduction lowers INR.

Check INR at 1, 2, and 4 weeks after starting or adjusting levothyroxine. A 10–25 % reduction in the warfarin dose is usually needed when the patient becomes euthyroid.