Is Levothyroxine (T4 thyroid replacement) Safe with Selenium?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
If you begin or stop selenium supplementation while taking levothyroxine, inform your prescriber. Thyroid function tests (TSH, free T3, free T4) should be rechecked within 6–8 weeks to assess whether your levothyroxine dose needs adjustment.
Why this interaction occurs
Selenium is a critical cofactor for type I and type II iodothyronine deiodinase enzymes responsible for peripheral conversion of T4 to active T3. Selenium supplementation (100–200 mcg/day) can increase T3 production, potentially causing relative over-replacement symptoms (palpitations, anxiety, weight loss) in patients on fixed levothyroxine doses.
Evidence
Evidence level: moderate.
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 5,677 reports mentioning Selenium. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the Selenium + 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 Levothyroxine (T4 thyroid replacement) interactions → · All Selenium interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.