Drug interactions
Fosphenytoin + Levothyroxine
Severity: Moderate
Fosphenytoin and Levothyroxine: 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
Fosphenytoin is converted to phenytoin, which increases levothyroxine clearance and can displace thyroid hormone from binding proteins. Hypothyroidism may emerge in patients on replacement therapy, and free hormone measurements can be misleading.
What this interaction means in plain words
Fosphenytoin and Levothyroxine 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.
Fosphenytoin belongs to Central Nervous System; Levothyroxine belongs to Endocrine.
Why it happens
Phenytoin induces hepatic metabolism of levothyroxine and competes for plasma protein binding.
What to do
Monitor TSH and free T4 during and after fosphenytoin therapy and raise the levothyroxine dose if hypothyroidism develops. Interpret total T4 cautiously because of binding displacement.
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
Other interactions of Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Levothyroxine?
MediHelp rates the interaction as moderate. Fosphenytoin and Levothyroxine 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. Fosphenytoin is converted to phenytoin, which increases levothyroxine clearance and can displace thyroid hormone from binding proteins. Hypothyroidism may emerge in patients on replacement therapy, and free hormone measurements can be misleading.
How serious is the interaction between Fosphenytoin and Levothyroxine?
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 during and after fosphenytoin therapy and raise the levothyroxine dose if hypothyroidism develops. Interpret total T4 cautiously because of binding displacement.
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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