Is Iodine (Kelp, Potassium Iodide) Safe with Levothyroxine?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Avoid iodine supplements and kelp products unless specifically prescribed by your endocrinologist. Do not self-supplement with iodine if you are on thyroid medication. Report any new supplement use to your thyroid care team.
Why this interaction occurs
Excess iodine can paradoxically inhibit thyroid hormone synthesis via the Wolff-Chaikoff effect and can trigger autoimmune thyroid flares in patients with Hashimoto's thyroiditis, the most common cause of hypothyroidism. In patients on levothyroxine, excess iodine can unpredictably alter TSH levels, cause thyroiditis, or worsen thyroid autoimmunity, making thyroid management unstable.
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, Potassium Iodide) interactions → · All Levothyroxine interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.