Food and medicine interaction
Seaweed and kelp supplements and antithyroid medicines such as methimazole (Tapazole)
ProbableEvidence level: probable. Case reports in people plus a known mechanism, but no controlled studies.
Last reviewed
What can happen
Seaweed can carry very large amounts of iodine, and NIH's Office of Dietary Supplements warns that high doses of iodine with antithyroid medicines can have an additive effect and could cause an underactive thyroid.
- Why it happens
- Excess iodine can itself slow thyroid hormone production, especially in people with thyroid disease, adding to the effect of the medicine.
- At ordinary food amounts
- Amounts vary hugely: ODS reports commercial seaweeds ranging from 16 to 2,984 mcg of iodine per gram, and lists 2 tablespoons of dried nori at 116 mcg. Kelp supplements and frequent seaweed dishes are the main concern.
What to do
If you take an antithyroid medicine, ask your prescriber before taking kelp supplements or eating seaweed often. Do not change your thyroid medicine on your own.
Never stop or change a prescribed medicine without talking to the prescriber.
Common questions
Can I have seaweed and kelp supplements while taking thyroid medicines?
If you take an antithyroid medicine, ask your prescriber before taking kelp supplements or eating seaweed often. Do not change your thyroid medicine on your own. Amounts vary hugely: ODS reports commercial seaweeds ranging from 16 to 2,984 mcg of iodine per gram, and lists 2 tablespoons of dried nori at 116 mcg. Kelp supplements and frequent seaweed dishes are the main concern.
Sources
Related
- RemedySeaweed: the full evidence pageNori, kombu, wakame and kelp are reliable sources of iodine, sometimes too reliable: the amount swings wildly between types, and concentrated kelp can push intake past the safe upper limit.
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