Dried, powdered bladderwrack (Fucus vesiculosus) supplies iodine, a mineral the thyroid uses to make its hormones. Most people stir the powder into water, juice, or a smoothie once a day with food. This seaweed supplement absorbs minerals from the ocean, so its iodine levels can vary from batch to batch.
Bladderwrack has a history of use for thyroid support, but solid evidence for other wellness claims is thin. The powder can be high in iodine. Too much iodine can upset the thyroid, especially if you already have a thyroid condition, so precise dosing requires care.
Ocean-grown seaweeds sometimes carry heavy metals. Get professional advice before using bladderwrack if you take thyroid medication or have a thyroid disorder. Skip it during pregnancy and breastfeeding unless a qualified clinician tells you otherwise.