Seaweed and Iodine

Seaweed is woven into Korean eating: toasted sheets of gim with rice, miyeok in soup and salads, and dashima in the broth behind countless stews. All three are natural sources of iodine, a mineral the thyroid gland needs to make its hormones. They also differ enormously in how much they contain, and that difference matters more than any general statement about seaweed being good or bad.

How much iodine do people need?

MedlinePlus gives the US Recommended Dietary Allowance as 150 micrograms (mcg) a day for adults and teenagers 14 and older, rising to 220 mcg in pregnancy and 290 mcg while breastfeeding. Upper limits differ by country. In the United States, the Tolerable Upper Intake Level for adults, including during pregnancy and breastfeeding, is 1,100 mcg a day. Korea's Ministry of Food and Drug Safety (MFDS) sets a higher adult upper limit of 2,400 mcg a day.

According to Korean dietary surveys cited in a 2025 study, the median Korean adult takes in about 352 mcg of iodine a day, and seaweed supplies roughly three quarters of all dietary iodine in the country.

Iodine in gim, miyeok and dashima

A 2025 study analyzed 348 seaweed samples gathered from 12 Korean coastal regions between 2020 and 2024. Measured on a dry-weight basis, the averages and ranges were:

• Gim (laver, Porphyra): about 100 mg per kg on average, ranging from roughly 23 to 412 mg per kg. That works out to about 100 mcg per gram of dried gim, the lowest of these three.
• Miyeok (sea mustard, Undaria): about 177 mg per kg on average, ranging from roughly 39 to 456 mg per kg.
• Dashima (sea tangle, Saccharina japonica): about 2,432 mg per kg on average, ranging from roughly 375 to 4,165 mg per kg. On average, a single gram of dried dashima held about as much iodine as Korea's daily adult upper limit.

The researchers also found that iodine varied with harvest time, region and whether the seaweed was wild or farmed, so any one package may sit far from the average. They note that boiling and blanching can remove a large share of the iodine, with published estimates ranging from 30 to 90 percent, which is relevant because dashima is usually simmered for stock and then lifted out. See making yuksu broth. Overall, the study judged the risk from typical Korean seaweed intake to be low against Korean reference values, but flagged dashima as exceeding stricter European and international benchmarks.

Why the thyroid cares

Too little iodine and too much can both disturb the thyroid. The Harvard T.H. Chan School of Public Health's Nutrition Source explains that excess iodine can push the gland toward overproduction (hyperthyroidism) in some people and suppress it (hypothyroidism) in others. MedlinePlus adds that very high iodine intake taken together with antithyroid medication can add to the drug's effect. Infants, children, older adults and people with existing thyroid disease, especially autoimmune thyroid disease, are described as the most sensitive.

Miyeok-guk after childbirth

In Korea, a new mother's first meal has traditionally been white rice with miyeok-guk, and the soup is also a fixture of birthday tables. The Encyclopedia of Korean Culture links the postpartum custom to a belief that seaweed supports breast milk, and records small rituals around it, such as tying rather than breaking the seaweed bought for a new mother.

The custom can add up to a lot of iodine. A 2011 analysis, as reported by later Korean researchers, estimated about 1,705 mcg of iodine in a 250 mL bowl of seaweed soup. A 2021 nationwide survey of 1,054 Korean women within eight weeks of giving birth found an average intake of about 2,946 mcg a day, falling below Korea's adult upper limit from about the fifth week. A small 2026 follow-up of 81 women found no thyroid disease one year after birth across intake levels, but the authors called the evidence inconclusive given the study's size and dropout. Women who are breastfeeding, and anyone with a thyroid condition, should talk with their clinician about how much seaweed soup makes sense.

Practical notes

• Variety over volume: A sheet of gim-gui or a side of miyeok-muchim is a modest amount; eating large amounts of dashima every day is a different matter.
• Kelp supplements and powders: Concentrated products can deliver far more iodine than food. Check with a clinician before using them.
• Sodium: Seasoned gim and seaweed side dishes can be salty. See sodium in Korean food.