Major
Levothyroxine sodium × Warfarin
Thyroid hormones×Vitamin K antagonist anticoagulants
Mechanism
Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine starts or its dose increases, warfarin sensitivity rises and INR climbs within 1–4 weeks.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool. Severe cases include gastrointestinal or intracranial haemorrhage.
Management
When levothyroxine starts or is escalated, check INR at 1, 2, and 4 weeks and titrate warfarin (typically a 10–25% dose reduction). Once thyroid function stabilises, warfarin requirements stabilise too.
Sources
- Lexicomp: Lexicomp Drug Interactions (2024)– Wolters Kluwer Clinical Drug Information, Inc. Lexi-Interact Online, 2024
- Pharmaceutical Press: Stockley's Drug Interactions, 12th edition (2024)– Preston CL (ed.). Stockley's Drug Interactions. 12th ed. London: Pharmaceutical Press; 2024