Drug interactions
Levothyroxine + Phenobarbital
Severity: Moderate
Levothyroxine and Phenobarbital: 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
Phenobarbital induces hepatic enzymes and can speed levothyroxine clearance. This may lower thyroid hormone levels and cause hypothyroid symptoms in patients on replacement therapy.
What this interaction means in plain words
Levothyroxine and Phenobarbital 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
Induction of hepatic metabolizing enzymes (including glucuronidation) increases levothyroxine clearance.
What to do
Monitor TSH and free T4 after starting or stopping phenobarbital, and increase the levothyroxine dose if 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 Levothyroxine
Frequently asked questions
Can I take Levothyroxine with Phenobarbital?
MediHelp rates the interaction as moderate. Levothyroxine and Phenobarbital 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. Phenobarbital induces hepatic enzymes and can speed levothyroxine clearance. This may lower thyroid hormone levels and cause hypothyroid symptoms in patients on replacement therapy.
How serious is the interaction between Levothyroxine and Phenobarbital?
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 phenobarbital, and increase the levothyroxine dose if needed.
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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