Bladder wrack is a seaweed that was the original source of iodine, discovered in 1812, and was used extensively to treat goiter, a swelling of the thyroid gland related to insufficient iodine. In the 1860s, it was claimed that bladderwrack, as a thyroid stimulant, could counter obesity by increasing metabolic rate, and, since then, it has been featured in numerous weight_loss remedies.

Bladderwrack is also known by the names Black Tang, Rockweed, Bladder Fucus, Seawrack, Sea Oak, Black Tany, Cut Weed, and Rockwrack. Bladderwrack is a type of seaweed found on the coasts of the North Sea, the western Baltic Sea, and the 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. Primary chemical constituents of this plant include mucilage, algin, mannitol, beta_carotene, zeaxanthin, iodine, bromine, potassium, volatile oils, and many other minerals. The main use of Bladderwrack (and other types of seaweed) in herbal medicine is as a source of iodine, an essential nutrient for the thyroid gland. Bladderwrack has proved most useful in the treatment of underactive thyroid glands (hypothyroidism) and goiter. Through the regulation of thyroid function, there is an improvement in all the associated symptoms. Where obesity is associated with thyroid trouble, this herb may be very helpful in reducing the excess weight. It has a reputation in helping the relief of rheumatism and rheumatoid arthritis, both used internally and as an external application upon inflamed joints. A chemical constituent 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 several over-the-counter preparations for heartburn. The same constituent gives Bladderwrack laxative properties as well. Other proposed uses of Bladderwrack include treating atherosclerosis and strengthening immunity, although there is no scientific evidence at present that it works for these purposes.

Bladderwrack should not be used in cases of hyperthyroidism or cardiac problems, or during pregnancy and lactation. Excessive dosage (many times the recommended dosage) may lead to hyperthyroidism, tremor, increased pulse rate and elevated blood pressure.

Bladder wrack on exposed shoreline