Is Soy Isoflavones Safe with Levothyroxine?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Avoid taking soy protein supplements, soy isoflavone capsules, or consuming large amounts of soy foods within 4 hours of your levothyroxine dose. Patients consuming consistent amounts of dietary soy should have TSH monitored more frequently to ensure adequate thyroid replacement.
Why this interaction occurs
Soy isoflavones (genistein, daidzein) inhibit thyroid peroxidase and can reduce the gastrointestinal absorption of levothyroxine. In hypothyroid patients, soy supplementation has been shown to impair levothyroxine bioavailability, requiring dose increases to maintain euthyroidism. The effect is most pronounced with soy-based supplements and protein isolates.
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 Soy Isoflavones interactions → · All Levothyroxine interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.