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Bladderwrack (Fucus vesiculosus)
A brown seaweed (Fucus vesiculosus) rich in iodine and the fiber fucoidan, marketed for thyroid support and weight loss. Human efficacy evidence is extremely thin, while the iodine-excess, heavy-metal and bleeding-risk concerns are real.
What the evidence says
Most Bladderwrack studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from medium-quality randomised trials published 1999–2024 with a typical study size of 12 participants.
Based on 5 studies · 2 RCTs · 80 total participants
Confidence
Low confidenceBy outcome
Direct human evidence for bladderwrack alone is essentially a 3-person case report; the one positive RCT used a Fucus+Ascophyllum+chromium combination, so no benefit can be attributed to bladderwrack itself.
No trials currently enrolling · 5 completed on ClinicalTrials.gov
Registered trials show research momentum for Bladderwrack, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed. They are never counted toward the evidence rating above.
Browse these trials on ClinicalTrials.govClinicalTrials.gov · as of Sep 2026
Bladderwrack is a brown seaweed traditionally used to make iodine supplements and folk thyroid remedies. Its two headline constituents are iodine (the substrate for thyroid hormone) and fucoidan (a sulfated-polysaccharide fiber with anticoagulant, anti-inflammatory and metabolic activity in lab studies).
Modern marketing pushes it for thyroid 'support,' weight loss and metabolic health.
The honest reality is that direct human evidence for bladderwrack is almost nonexistent: the most-cited human report is a case series of just three women, and the one randomized trial showing a glycemic benefit used Fucus combined with Ascophyllum AND chromium, so the effect cannot be attributed to bladderwrack alone.
Meanwhile the safety concerns are concrete — like all brown seaweeds it carries a variable, often excessive iodine load that can disrupt the thyroid, it bioaccumulates arsenic and other heavy metals from seawater, and fucoidan has measurable anticoagulant activity in animal models, raising a plausible bleeding-risk interaction.
There is no validated efficacy dose, and pregnancy use should be avoided.
Concentrates iodine, the substrate for thyroid hormone synthesis — in variable and often excessive amounts.
Sulfated fiber with anticoagulant, anti-inflammatory and enzyme-inhibiting activity in preclinical models; clinical relevance in humans is unproven.
Brown-seaweed polyphenols inhibit carbohydrate-digesting enzymes in vitro, a proposed basis for blunting glucose spikes.
How Bladderwrack works — from molecular targets to health outcomes. Click an edge to see supporting research.This visualization is in beta — pathways are being refined and expanded.
Avoid — unpredictable iodine load, heavy-metal risk, and potential hormonal/anti-estrogenic effects.
Avoid — excess iodine can trigger hyper- or hypothyroidism.
Avoid — fucoidan's anticoagulant activity may add bleeding risk.
Fucoidan has measurable anticoagulant (antithrombin / anti-factor Xa) activity in animal models — plausible additive bleeding risk.
Variable iodine load can destabilize thyroid control and interfere with thyroid drug dosing.
Drugs that already perturb thyroid function; added iodine compounds the risk.
Tip: Avoid if you have thyroid disease; do not use as a chronic high-dose iodine source
Tip: Avoid with anticoagulants and before surgery
Tip: Use only third-party-tested products
The current evidence for Bladderwrack is insufficient to assign an evidence score, based on 5 indexed studies. A brown seaweed (Fucus vesiculosus) rich in iodine and the fiber fucoidan, marketed for thyroid support and weight loss. Human efficacy evidence is extremely thin, while the iodine-excess, heavy-metal and bleeding-risk concerns are real. Representative study: PMID 30714233.
The commonly studied dose of Bladderwrack is No validated efficacy dose; iodine content is unpredictable, so the delivered iodine dose is hard to control. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
The best time to take Bladderwrack is with meals. Take it with food. Dosing is effectively framed by iodine content (adult RDA 150 mcg, upper limit ~1100 mcg/day), but products vary so widely that the actual dose is uncertain.
Bladderwrack should be used with caution — talk to a healthcare provider before taking it. Reported side effects are uncommon and include thyroid dysfunction (hyper- or hypothyroidism) from iodine excess, heavy-metal (arsenic) exposure, increased bleeding/bruising (fucoidan anticoagulant effect). Use caution if any of these apply to you: Pregnancy and breastfeeding; Existing thyroid disease; Bleeding disorders or upcoming surgery.
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Reviewed by Dr. Baher Al Hakim · Last reviewed June 2026 · evidence from 5 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.
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