Drug interactions
Levothyroxine + Phenytoin
Severity: Moderate
Levothyroxine and Phenytoin: 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
Phenytoin can lower circulating thyroid hormone levels and may increase levothyroxine requirements. Patients with hypothyroidism may become under-replaced.
What this interaction means in plain words
Levothyroxine and Phenytoin 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.
Levothyroxine belongs to Endocrine; Phenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces hepatic metabolism of thyroid hormones and may displace them from binding proteins.
What to do
Monitor TSH and free T4 after starting or stopping phenytoin. Increase the levothyroxine dose as needed.
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 Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Other interactions of Levothyroxine
Frequently asked questions
Can I take Levothyroxine with Phenytoin?
MediHelp rates the interaction as moderate. Levothyroxine and Phenytoin 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. Phenytoin can lower circulating thyroid hormone levels and may increase levothyroxine requirements. Patients with hypothyroidism may become under-replaced.
How serious is the interaction between Levothyroxine and Phenytoin?
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 TSH and free T4 after starting or stopping phenytoin. Increase the levothyroxine dose as needed.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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