Is Iodine (Kelp, Seaweed Supplements) Safe with Levothyroxine?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Avoid high-dose iodine supplements, kelp, and bladderwrack supplements while on thyroid medications. Do not take iodine supplements without physician guidance. The Wolff-Chaikoff effect can cause paradoxical hypothyroidism with excessive iodine, counteracting your thyroid medication.
Why this interaction occurs
Excessive iodine intake from supplements like kelp or iodine drops can paradoxically inhibit thyroid hormone synthesis (Wolff-Chaikoff effect) and can cause both hypothyroidism and hyperthyroidism depending on the patient's underlying thyroid status. In patients on stable levothyroxine, high iodine can worsen autoimmune thyroid disease (Hashimoto's) and destabilize TSH control.
Evidence
Evidence level: strong.
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 Iodine (Kelp, Seaweed Supplements) interactions → · All Levothyroxine interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.