Warfarin and Levothyroxine: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Warfarin
Levothyroxine
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Increased response to warfarin therapy
Interaction Deep Dive
Concomitant use of levothyroxine and oral anticoagulants increases the response to oral anticoagulant therapy312. However, a small retrospective study did not find a significant change in the mean warfarin dose/INR ratio during 90 days of follow up 4. A decrease in the dose of anticoagulant may be warranted with correction of the hypothyroid state or when the levothyroxine dose is increased. Closely monitor prothrombin time 2 or other coagulation tests to allow for appropriate and timely dosage adjustments 31.
Why it happens (mechanism)
An unknown mechanism
Literature reports
2 reports — tap to read
a) In a retrospective study in adults receiving stable warfarin (N=102; mean age 73.9 years; 51.3% with atrial fibrillation or flutter), initiation of levothyroxine resulted in no significant change in the mean warfarin dose/INR ratio during a 90-day followup (12.9 preexposure vs 13.5 postexposure). The mean warfarin dose/INR ratio was also not significantly different between pre and post exposure in patients who achieved euthyroidism following concomitant use (13.42 preexposure vs 12.7 postexposure) 4.
b) Elderly patients on concomitant levothyroxine and warfarin therapy did not have an increased risk of hospitalization due to hemorrhage compared with controls. The risk of hemorrhage due to the concomitant use of levothyroxine was evaluated in a 16-year population-based nested case-control analysis involving older patients (n=10,532) with a median age of 80 years (interquartile range, 75 to 86 years) who received warfarin for at least 3 months and were hospitalized due to hemorrhage (case patients). Control patients were on warfarin and were not hospitalized for hemorrhage (n=40,693). There was a nonsignificant 11% increased risk of hemorrhage in case patients initiating levothyroxine within 30 days of the date of the first hospital admission for hemorrhage. There was also no significant association with more distant exposures for case patients initiating levothyroxine 31 to 60 days (decrease 24%) or 61 to 90 days (decreased 33%) prior to the index date 5.
Common questions
Can I take Warfarin and Levothyroxine together?
Increased response to warfarin therapy Always confirm with your pharmacist or prescriber before making any change.
How serious is the Warfarin and Levothyroxine interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
- Are Blueberries Safe To Eat If You Take Warfarin?
- Does Ashwagandha Interact With Warfarin?
- Does Flaxseed Interact With Warfarin?
- Levothyroxine 88 mcg Tablets Changed From Green to Lavender
- Black Currant Oil With Levothyroxine, Methotrexate, and a Statin
- Provitalize With Levothyroxine, Valacyclovir, and Citalopram
Questions for your pharmacist
- Does my dose of Warfarin or Levothyroxine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (5)
- Product Information: TIROSINT(R)-SOL oral solution, levothyroxine sodium oral solution. IBSA Pharma Inc. (per FDA), Parsippany, NJ, 2023. DailyMed
- Product Information: levothyroxine sodium injection powder for solution, levothyroxine sodium injection powder for solution. Fresenius Kabi USA, LLC, Lake Zurich, IL, 2017. DailyMed
- Product Information: SYNTHROID(R) oral tablets, levothyroxine sodium oral tablets. AbbVie Inc (per FDA), North Chicago, IL, 2024. DailyMed
- Wood MD, Delate T, Clark M, et al: An evaluation of the potential drug interaction between warfarin and levothyroxine. J Thromb Haemost 2014; 12(8):1313-1319. PubMed
- Pincus D, Gomes T, Hellings C, et al: A population-based assessment of the drug interaction between levothyroxine and warfarin. Clin Pharmacol Ther 2012; 92(6):766-770. PubMed
Keep reading about Warfarin
Keep reading about Levothyroxine
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Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
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