Bladderwrack is a type of seaweed found on the coasts of the North Sea, the western Baltic Sea, and the Atlantic and Pacific Oceans. A common food in Japan, it is used as an additive and flavoring in various food products in Europe. Bladderwrack is commonly found as a component of kelp tablets or powders used as nutritional supplements. It is sometimes loosely called kelp, but that term technically refers to a different seaweed.

Bladderwrack contains high concentrations of iodine, and for this reason it has been recommended as a treatment for hypothyroidism(underactive thyroid gland).1 However, iodine will only help for the type of hypothyroidism caused by iodine deficiency, which is a relatively rare condition in the developed world. If your iodine levels are not low, taking extra amounts of iodine can cause your thyroid gland to become either over- or underactive, causinghyperthyroidism or hypothyroidism, respectively. Furthermore, the amount of iodine supplied by bladderwrack is unpredictable.

A component of bladderwrack called alginic acid swells upon contact with water. When taken orally, it forms a type of seal at the top of the stomach, and for this reason is used in over-the-counter preparations for heartburn. The same constituent gives bladderwrack laxative properties, as well.2

Other proposed uses of bladderwrack include treating atherosclerosis and strengthening immunity, but there is no meaningful evidence at present that it works for these purposes.3,4