Kombu seaweed and thyroid function in Japanese adults
What it found
Eating 15 or 30 g of kombu seaweed daily raised a key thyroid hormone signal in healthy Japanese adults.
The changes went back to normal within 7 to 40 days after they stopped eating it.
What they found
Studies in people
Short-term thyroid changes
Eating 15 or 30 g of kombu daily for 7 to 10 days raised TSH levels in the blood. In some people these levels went above the normal range. Other thyroid hormones dropped slightly but stayed within normal limits.
Long-term thyroid changes
Eating 15 g of kombu daily for 55 to 87 days kept TSH levels high. The other thyroid hormones stayed about the same as before. The slowing effect lasted for at least 3 months.
Iodine in urine
Urine iodine rose to about 30 times the starting level with 15 g of kombu. With 30 g it rose to about 44 times the starting level. This showed the body took in far more iodine than the recommended upper limit of 3 mg per day.
Recovery after stopping
All unusual test results went back to their starting levels 7 to 40 days after people stopped eating the kombu. The authors say the slowing effect was slight and reversible.
What the authors conclude
- They found that eating kombu slowed down thyroid function in healthy Japanese adults, though the effect went away after stopping.
- The authors recommend that Japanese people avoid eating very large amounts of seaweed.
- They point out that the absorbed iodine greatly exceeded the recommended upper limit of 3 mg per day.
How it was done
Japan is an area where people get plenty of iodine, mostly from seaweed. Too much iodine can slow down the thyroid gland, so the authors wanted to see if eating kombu affects thyroid function in healthy adults.
Thirteen healthy Japanese adults, aged around 27 on average, took part in 23 tests. They ate 15 or 30 g of kombu seaweed every day for 7 to 10 days, or 15 g daily for 55 to 87 days. Researchers measured their thyroid hormones and iodine levels in blood and urine before, during and after.
What it can’t tell you
- This trial only included 13 healthy Japanese adults who already eat iodine-rich foods.
- It cannot show what would happen in people from other countries or in people with existing thyroid problems.
Who paid
- Authors work at
- Koshien University, Japan
The paper
- Title
- Suppression of thyroid function during ingestion of seaweed "Kombu" (Laminaria japonoca) in normal Japanese adults
- Type
- Clinical trial
- Evidence
- Studies in people
- Summarised from
- Full text
- Cite
- Miyai K, Tokushige T, Kondo M, Iodine Research Group (2008). Suppression of thyroid function during ingestion of seaweed "Kombu" (Laminaria japonoca) in normal Japanese adults. Endocrine journal. doi:10.1507/endocrj.k08e-125Free full textPubMed 18689954DOI
Summary written 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error