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Drug Interactions

warfarin and levothyroxine

MajorAvoid this combination where possible.

Mechanism
Thyroid hormones increase the turnover of vitamin K-dependent clotting factors. Correcting hypothyroidism with levothyroxine therefore potentiates the anticoagulant effect of warfarin, while withdrawal of levothyroxine reduces it.
What it means clinically
Patients may experience an increased INR and risk of bleeding after starting levothyroxine, or reduced anticoagulation after stopping it. This can lead to clinically significant bleeding or thromboembolism if not managed.
What to do
Monitor INR closely (e.g., weekly) when levothyroxine is initiated, discontinued, or the dose is changed. Adjust the warfarin dose as needed to maintain the target INR. Advise patients to report signs of bleeding.
Evidence
Well-established and widely documented in clinical guidelines and prescribing information. Requires routine monitoring.
The two drugs
warfarinVitamin K antagonistRead the monographlevothyroxineThyroid hormoneRead the monograph

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Other interactions involving warfarin or levothyroxine

Educational reference for students and clinicians in training. It does not replace professional judgement, local protocols or current prescribing information.