Is Selenium Safe with Levothyroxine?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Do not self-supplement selenium above the recommended daily allowance (55 mcg/day) without medical supervision. If selenium is recommended for autoimmune thyroid disease, have TSH monitored as levothyroxine dosing may need adjustment.
Why this interaction occurs
Selenium is essential for the enzyme deiodinase, which converts T4 (levothyroxine) to the active T3. Selenium supplementation in deficient patients may increase T4-to-T3 conversion efficiency, potentially altering the effective dose of levothyroxine needed. Excess selenium is also toxic.
Evidence
Evidence level: moderate.
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 302,514 reports mentioning Levothyroxine. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the Levothyroxine + supplement combination on this page.
FAERS reports are voluntary and cannot prove that a drug caused an event. Do not use report counts to assess the safety of this specific drug-supplement pair.
Frequently asked questions
Related interactions
All Selenium interactions → · All Levothyroxine interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.