It is important to be aware that it is easy to take in an excessive amount of iodine through eating seaweed, particularly if it is one of the brown seaweeds like kelp or kombu. Kelp supplements have highly variable iodine content with most UK kelp supplements providing more than the content declared on the label. It is very important to avoid excessive iodine as this can damage the thyroid causing under- and over-active thyroid disease. There is information and advice about seaweed consumption and excessive iodine intake in our factsheet.
Some studies have found that eating high amounts of seaweed eating can lead to underactive thyroid disease (hypothyroidism), including hypothyroidism in new-borns if the mother ate excessive seaweed during pregnancy. Other studies have linked high seaweed intake to over-active thyroid disease (hyperthyroidism) and thyroid cancer. For example, a large Japanese study found that in women after the menopause there was a greater risk of thyroid cancer in those who consumed seaweed almost daily than those who consumed it twice a week or less.
Kelp and seaweed supplements are not recommended during pregnancy. The Food Standards Agency of Australia and New Zealand advises pregnant and lactating women and children not to eat more than one meal per week containing brown seaweed.