Drug interactions
Levothyroxine + Phenindione
Severity: Moderate
Levothyroxine and Phenindione: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors, which strengthens the effect of oral anticoagulants. Starting or increasing levothyroxine in a patient on phenindione may raise INR and bleeding risk.
What this interaction means in plain words
Levothyroxine and Phenindione can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Why it happens
Increased catabolism of vitamin K-dependent clotting factors enhances the anticoagulant effect.
What to do
Monitor INR closely when levothyroxine is started, stopped or its dose changed, and adjust the phenindione dose accordingly.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Levothyroxine
Frequently asked questions
Can I take Levothyroxine with Phenindione?
MediHelp rates the interaction as moderate. Levothyroxine and Phenindione can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors, which strengthens the effect of oral anticoagulants. Starting or increasing levothyroxine in a patient on phenindione may raise INR and bleeding risk.
How serious is the interaction between Levothyroxine and Phenindione?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor INR closely when levothyroxine is started, stopped or its dose changed, and adjust the phenindione dose accordingly.
What other medicines interact with Levothyroxine?
MediHelp also lists interactions of Levothyroxine with, for example, Cholestyramine, Sodium iodide i-131, Tiratricol. The full list is on the Levothyroxine page.
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