Does Levothyroxine Interact With Phenytoin?

Phenytoin and carbamazepine reduce serum protein binding of levothyroxine, and total and free T4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.

Severity: Minor

Severity basis: drug interactions section. Combining Levothyroxine (Thyroid hormone) and Phenytoin (Anticonvulsant) is classified as a minor interaction.

What to Do

This is generally a minor interaction, but review it with your pharmacist to be sure it is safe for you.

From the FDA Label

Phenytoin and carbamazepine reduce serum protein binding of levothyroxine, and total and free T4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.

Source: FDA structured product labeling (set_id 008de8fd-150f-4022-8ccb-c8bfa94875c7).

Levothyroxine

Thyroid hormone

Brands: Synthroid, Levoxyl, Euthyrox

View Levothyroxine safety profile

Phenytoin

Anticonvulsant

Brands: Dilantin

View Phenytoin safety profile

Check these against your full medication list in our free Interaction Checker

This information is educational — not medical advice.

This page is provided for general educational purposes and summarizes publicly available data from sources such as the U.S. Food & Drug Administration. It is not a substitute for the judgment of a licensed clinician and should not be used to start, stop, or change any medication. It may be incomplete or out of date, and individual circumstances vary. Always talk with your prescriber or pharmacist about your specific medications and health conditions. If you think you may have a medical emergency, call 911.

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