Liothyronine and Warfarin: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Warfarin
Liothyronine
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
An increased risk of bleeding
Interaction Deep Dive
The hypoprothrombinemic response to oral anticoagulants is increased in hyperthyroidism and blunted in hypothyroidism. Thyroid replacement in a patient with hypothyroidism may increase metabolism of clotting factors, thus increasing the anticoagulant effects891. However, in a retrospective study in adults receiving stable warfarin (N=102), initiation of levothyroxine resulted in no significant change in the mean warfarin dose/INR ratio during a 90-day followup 2. Additionally, in a large study in elderly patients on concomitant levothyroxine and warfarin therapy there was no significant increased risk of hospitalization due to hemorrhage compared with controls (n=40,693) 3. Additional monitoring may not be necessary.
Why it happens (mechanism)
Increased metabolism of vitamin K-dependent clotting factors
Literature reports
4 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) 2.
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 3.
c) Thyroid hormone preparations increase the catabolism of vitamin K-dependent clotting factors. Thus, if thyroid hormones are initiated in patients already stabilized on oral anticoagulants, enhanced responses to the anticoagulant are to be expected, and downward anticoagulant dosage adjustments will usually be necessary. On the other hand, patients already stabilized on thyroid hormone products such that they are euthyroid will respond normally to the introduction of anticoagulant therapy, and no special precautions or dosage adjustments are expected 1456.
d) Seven patients (age range, 35 to 60 years) who had myxedema were found to have a diminished hypoprothrombinemic response to warfarin during the myxedematous stage. This disappeared after initiation of liothyronine therapy in an oral dose of 50 to 100 mcg daily 7.
Common questions
Can I take Liothyronine and Warfarin together?
An increased risk of bleeding Always confirm with your pharmacist or prescriber before making any change.
How serious is the Liothyronine and Warfarin interaction?
It is rated minor. Usually limited clinical impact.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "established". Well documented — supported by controlled studies or strong clinical data.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Liothyronine or Warfarin 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 (9)
- Hansten PD: Oral anticoagulants and drugs which alter thyroid function. Drug Intell Clin Pharm 1980; 14:331-334. DOI
- 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. DOI
- Self TH, Straughn AB, & Weisburst MR: Effect of hyperthyroidism on hypoprothrombinemic response to warfarin. Am J Hosp Pharm 1976; 33:387-389. DOI
- Self T, Weisburst M, Wooten E, et al: Warfarin-induced hypoprothrombinemia. Potentiation by hyperthyroidism. JAMA 1975; 231:1165-1166. DOI
- Vagenakis AG, Cote R, Miller ME, et al: Enhancement of warfarin-induced hypoprothrombinemia by thyrotoxicosis. John Hopkins Med J 1972; 131:69-73.
- Rice AJ, McIntosh TJ, Fouts JR, et al: Decreased sensitivity to warfarin in patients with myxedema. Am J Med Sci 1971; 262:211-215. PubMed
- Stephens MA, Self TH, Lancaster D, et al: Hypothyroidism: effect on warfarin anticoagulation. South Med J 1989; 82:1585-1586.
- Costigan DC, Freedman MH, & Ehrlich RM: Potentiation of oral anticoagulant effect of L-thyroxine. Clin Pediatr 1984; 23:172-174.
Keep reading about Warfarin
Keep reading about Liothyronine
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