Warfarin with Thyroid Interaction Details
Brand Names Associated with Warfarin
- Coumadin®
- Jantoven®
- Warfarin
Brand Names Associated with Thyroid
- Armour® Thyroid
- Desiccated thyroid
- Thyroid
- thyroid extract
- thyroid gland
Medical Content Editor Dr. Brian Staiger, PharmD
Last updated
Nov 07, 2023
Interaction Effect
An increased risk of bleeding
Interaction Summary
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 effects. 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 . 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) . Additional monitoring may not be necessary.
Severity
Minor
Onset
Delayed
Evidence
Established
How To Manage Interaction
In patients receiving oral anticoagulant therapy, the prothrombin time or international normalized ratio (INR) may need to be monitored with the addition and withdrawal of treatment with thyroid hormones. However, in evidence from studies, additional monitoring may not be necessary . 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 are necessary .
Mechanism Of Interaction
Increased metabolism of vitamin K-dependent clotting factors
Literature Reports
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) .
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 .
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 .
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 .
Warfarin Overview
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Warfarin is used to prevent blood clots from forming or growing larger in your blood and blood vessels. It is prescribed for people with certain types of irregular heartbeat, people with prosthetic (replacement or mechanical) heart valves, and people who have suffered a heart attack. Warfarin is also used to treat or prevent venous thrombosis (swelling and blood clot in a vein) and pulmonary embolism (a blood clot in the lung). Warfarin is in a class of medications called anticoagulants ('blood thinners'). It works by decreasing the clotting ability of the blood.
Thyroid Overview
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Thyroid is used to treat the symptoms of hypothyroidism (a condition where the thyroid gland does not produce enough thyroid hormone). Symptoms of hypothyroidism include lack of energy, depression, constipation, weight gain, hair loss, dry skin, dry coarse hair, muscle cramps, decreased concentration, aches and pains, swelling of the legs, and increased sensitivity to cold. Thyroid is also used to treat goiter (enlarged thyroid gland). Thyroid is in a class of medications called thyroid agents. It works by supplying the thyroid hormone normally produced by the body.
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Definitions
Severity Categories
Contraindicated
These drugs, generally, should not be used together simultaneously due to the high risk of severe adverse effects. Combining these medications can lead to dangerous health outcomes and should be strictly avoided unless otherwise instructed by your provider.
Major
This interaction could result in very serious and potentially life-threatening consequences. If you are taking this drug combination, it is very important to be under close medical supervision to minimize severe side effects and ensure your safety. It may be necessary to change a medication or dosage to prevent harm.
Moderate
This interaction has the potential to worsen your medical condition or alter the effectiveness of your treatment. It's important that you are monitored closely and you potentially may need to make adjustments in your treatment plan or drug dosage to maintain optimal health.
Minor
While this interaction is unlikely to cause significant problems, it could intensify side effects or reduce the effectiveness of one or both medications. Monitoring for changes in symptoms and your condition is recommended, and adjustments may be made if needed to manage any increased or more pronounced side effects.
Onset
Rapid: Onset of drug interaction typically occurs within 24 hours of co-administration.
Delayed: Onset of drug interaction typically occurs more than 24 hours after co-administration.
Evidence
Level of documentation of the interaction.
Established: The interaction is documented and substantiated in peer-reviewed medical literature.
Theoretical: This interaction is not fully supported by current medical evidence or well-documented sources, but it is based on known drug mechanisms, drug effects, and other relevant information.
How To Manage The Interaction
Provides a detailed discussion on how patients and clinicians can approach the identified drug interaction as well as offers guidance on what to expect and strategies to potentially mitigate the effects of the interaction. This may include recommendations on adjusting medication dosages, altering the timing of drug administration, or closely monitoring for specific symptoms.
It's important to note that all medical situations are unique, and management approaches should be tailored to individual circumstances. Patients should always consult their healthcare provider for personalized advice and guidance on managing drug interactions effectively.
Mechanism Of Interaction
The theorized or clinically determined reason (i.e., mechanism) why the drug-drug interaction occurs.
Disclaimer: The information provided on this page is for informational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional regarding your specific circumstances and medical conditions.
Where Does Our Information Come From?
Information for our drug interactions is compiled from several drug compendia, including:
The prescribing information for each drug, as published on DailyMED, is also used.
Individual drug-drug interaction detail pages contain references specific to that interaction. You can click on the reference number within brackets '[]' to see what reference was utilized.
The information posted is fact-checked by HelloPharmacist clinicians and reviewed quarterly.