Is Soy Isoflavones Safe with Synthroid?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Separate soy-based supplements or high-soy foods from Synthroid by at least 4 hours. Monitor TSH more frequently if consuming soy isoflavones regularly. Infants on soy formula require careful Synthroid dose monitoring.
Why this interaction occurs
Soy isoflavones (genistein, daidzein) can bind to levothyroxine in the GI tract and reduce its absorption. Additionally, isoflavones inhibit thyroid peroxidase, an enzyme critical to endogenous thyroid hormone synthesis, potentially increasing total thyroid hormone requirements. This interaction is clinically significant in infants with congenital hypothyroidism on soy formula, who may require substantially higher levothyroxine doses.
Evidence
Evidence level: strong.
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 185,542 reports mentioning Synthroid. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the Synthroid + 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 Synthroid interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.