Bladderwrack
FucaceaeFucus vesiculosus
Also known as: Kelp, Rockweed, Black Tang
clinical_notes Clinical Summary
Bladderwrack (Fucus vesiculosus) is a brown seaweed historically used for iodine-deficient hypothyroidism, goiter, and obesity since its role in the 1811 discovery of iodine.
Its active constituents include variable levels of iodine, fucoidan (anti-inflammatory sulfated polysaccharide), alginates (demulcent), and fucoxanthin (antioxidant carotenoid).
Caution is warranted due to unpredictable iodine content and heavy-metal bioaccumulation risk; it is contraindicated in hyperthyroidism and requires thyroid monitoring during use.
Pregnancy Safety
Use cautiously and only at dietary doses (maximum ~300 mcg iodine/day). Excessive iodine from supplements can cause transient neonatal hypothyroidism. Heavy metal contamination concerns.
Lactation Safety
Iodine transfers into breast milk. High-dose supplementation may cause infant thyroid dysfunction. Dietary amounts likely safe; avoid concentrated supplements.
warning Contraindications
- Hyperthyroidism / Graves disease (contraindicated)Clinically Proven
- Concurrent levothyroxine therapy (caution)Clinically Proven
- Pregnancy (caution)Clinically Proven
- Anticoagulant therapy (warfarin, heparin, DOACs) (caution)Theoretical
- Iodine allergy/sensitivity (avoid)Clinically Proven
vital_signs Clinical Profile
Primary Indications
- check_circle iodine deficiency
- check_circle hypothyroidism (iodine-deficient)
- check_circle obesity support
- check_circle goiter
- check_circle heartburn (alginates)
Therapeutic Actions
System Affinities
- check_circle thyroid
- check_circle metabolic system
- check_circle skin
- check_circle digestive tract
labs Active Constituents
iodine
fucoidan
alginic acid
laminarin
fucoxanthin
phlorotannins
mucopolysaccharides
mannitol
selenium
history_edu Traditional Use
Traditional Chinese Medicine (TCM)
墨角藻 (Mo Jiao Zao)
Nature: cold
- softens hardness
- disperses nodules
- clears phlegm-heat
- promotes urination
Not native to traditional TCM materia medica but overlaps functionally with Kun Bu (Laminaria) and Hai Zao (Sargassum) for thyroid and nodule conditions.
Traditional Uses Across Healing Systems
While many herbs lack controlled clinical trials, centuries of traditional practice across cultures provide valuable insight into their therapeutic applications.
Iodine-deficient goiter, hypothyroidism, obesity, rheumatism.
Discovery of iodine in bladderwrack ash led to identification of iodine as essential thyroid nutrient.
Used by coastal Indigenous peoples of North America as food and poultice for joint and skin complaints.
Abundant coastal food source.
spa Parts Used
thallus (whole frond)
- iodine deficiency
- hypothyroidism
- heartburn
- weight management
Dried whole thallus. Iodine content varies widely (16-8000 mcg/g) depending on harvest location; standardization important. Heavy metal testing critical.
shield Safety
Contraindications — Evidence Basis
Hyperthyroidism / Graves disease
Iodine content can exacerbate hyperthyroidism; multiple case reports of iodine-induced thyrotoxicosis.
Concurrent levothyroxine therapy
Iodine may alter thyroid hormone requirements; also mucopolysaccharides may reduce levothyroxine absorption.
Pregnancy
Excess iodine >2-3 g/day associated with neonatal transient hypothyroidism. Heavy metal contamination risk (arsenic, mercury) of particular concern in pregnancy.
Anticoagulant therapy (warfarin, heparin, DOACs)
Fucoidan has in-vitro antithrombin and anticoagulant activity; may increase bleeding risk.
Iodine allergy/sensitivity
Can trigger hypersensitivity reactions in iodine-allergic individuals.
Monitoring Parameters
Monitor during use, especially with prolonged or high-dose therapy.
TSH, free T4, free T3
Baseline, at 4-6 weeks, then every 3 monthsIodine content can shift thyroid function in either direction (Wolff-Chaikoff or Jod-Basedow effect)
flagThreshold: TSH <0.4 or >5.0 mIU/L: discontinue and refer to endocrinology
Heavy metal panel (As, Hg, Pb, Cd)
Baseline if chronic use planned; every 12 months if continuousBrown algae bioaccumulate heavy metals; case of kidney failure linked to contaminated product
flagThreshold: Any elevated metals: discontinue, source-test product
Toxicity
Chronic ingestion of >1.1 mg iodine/day may cause thyroid dysfunction; heavy metal toxicity possible from contaminated sources.
Iodine-induced hyper- or hypothyroidism; acne; metallic taste; heavy metal toxicity (nephrotoxicity, neurological symptoms); reported case of renal failure.
Discontinue; thyroid function testing; heavy metal screening if exposure suspected; supportive care.
Adverse Effects
CYP Metabolism
Fucophlorethols from Fucus vesiculosus show in vitro inhibition of select CYP450 enzymes; clinical significance unclear. Potential to alter absorption of strontium, iron, and thyroid medications via mucopolysaccharide binding.
swap_horiz Interactions
Levothyroxine
Class: Thyroid hormone
Fucus vesiculosus contains variable and often very high iodine concentrations (up to 600 mg/g, with U.S.D.A. phytochemical estimate ~5.4 mg/g); excess iodine can induce the Wolff-Chaikoff effect, precipitate hypothyroidism in autoimmune thyroiditis, or precipitate hyperthyroidism (Jod-Basedow) in nodular goiter, de-stabilising levothyroxine dosing.
Avoid chronic bladderwrack supplementation in patients on thyroid replacement. If used short-term, re-check TSH and free T4 every 6 weeks and adjust levothyroxine dose.
Methimazole
Class: Antithyroid agent
High iodine load from bladderwrack provides substrate for thyroid hormone synthesis, antagonising the effect of thionamides that inhibit thyroid peroxidase; can worsen Graves' hyperthyroidism.
Contraindicated in patients on methimazole, propylthiouracil or awaiting radioiodine therapy. Screen thyroid patients for kelp/seaweed supplement use.
Amiodarone
Class: Class III antiarrhythmic
Amiodarone contains ~37% iodine by weight and blocks T4→T3 conversion. Adding bladderwrack iodine load increases total iodine exposure and risk of amiodarone-induced thyroid dysfunction and pulmonary toxicity.
Contraindicated with amiodarone. Screen amiodarone patients specifically for kelp/bladderwrack supplements. Monitor TSH every 3 months on amiodarone.
Lithium carbonate
Class: Mood stabilizer
A 60-year-old man on lithium developed hyperthyroidism after taking Fucus vesiculosus; iodine from bladderwrack counteracted the usual anti-thyroid effect of lithium, precipitating thyrotoxicosis.
Avoid bladderwrack in lithium-treated patients; re-screen thyroid function in any lithium patient presenting with symptoms of hyperthyroidism after herbal use.
Warfarin
Class: Anticoagulant
Fucoidan, a sulphated polysaccharide in bladderwrack, has anti-thrombin activity via heparin-cofactor II and antithrombin III, producing heparin-like anticoagulation that is additive with warfarin.
Monitor INR closely when bladderwrack is started; stop bladderwrack at least 2 weeks before elective surgery to minimise bleeding risk.
hub Combinations
Synergistic pairings can enhance therapeutic outcomes, while knowing suitable substitutes helps when specific herbs are unavailable or contraindicated.
Synergistic Combinations
4Ashwagandha
Limited EvidenceAshwagandha supports T4→T3 conversion and HPT axis regulation while bladderwrack provides iodine substrate.
Small RCTs show ashwagandha improves thyroid function; combination common in naturopathic protocols.
Bacopa
Limited EvidenceBoth in naturopathic thyroid protocols; Bacopa enhances cognitive symptoms of hypothyroidism while bladderwrack addresses iodine deficiency.
Clinical protocols; limited controlled trials of combination.
Self-Heal
Traditional UseSelf-heal (Xia Ku Cao) traditionally softens hardness and disperses thyroid nodules; complementary to iodine supplementation via bladderwrack.
TCM-Western integrative thyroid formulas.
Stinging Nettle
Traditional UseNettle supplies iron and minerals; bladderwrack supplies iodine — both part of mineral-rich tonic formulas.
Traditional formula combination.
science Studies
The Effect of Sargassum fusiforme and Fucus vesiculosus on Continuous Glucose Levels in Overweight Patients with Type 2 Diabetes Mellitus: A Feasibility Randomized, Double-Blind, Placebo-Controlled Trial
RCTThis feasibility randomised, double-blind, placebo-controlled pilot trial enrolled 36 overweight patients with type 2 diabetes and assigned them to 5 g/day dried Sargassum fusiforme, 5 g/day dried Fucus vesiculosus, or 0.5 g/day dried Porphyra (control) for 5 weeks alongside regular diabetes treatment. Primary outcome was weekly average blood glucose via continuous glucose monitoring. The daily intake of F. vesiculosus had no differential effect on weekly average blood glucose levels compared to control. Secondary outcomes (anthropometrics, plasma lipids, dietary intake) showed no significant changes. This feasibility study suggests 5 g/day natural F. vesiculosus is insufficient to meaningfully alter glycaemic control in T2DM.
Effects of fucoidan from Fucus vesiculosus in reducing symptoms of osteoarthritis: a randomized placebo-controlled trial
RCTThis double-blind, randomised, placebo-controlled trial evaluated a 300 mg daily dose of Fucus vesiculosus extract (85% fucoidan) over 12 weeks in 122 patients with mild-to-moderate osteoarthritis of the hip or knee, measured using the validated Comprehensive Osteoarthritis Test (COAT). Ninety-six participants completed the study. The reduction in OA symptoms was not significantly different from the placebo response, meaning the primary efficacy endpoint was not met. However, the extract was safe and well tolerated with no clinically significant changes in cholesterol, liver function, renal function, or haematopoietic markers. This study provides safety data but does not support a single-agent fucoidan dose for OA symptom relief at the tested dose.
medication Dosing
capsule
500-1000 mg dried thallus (providing 150-300 mcg iodine)
1-2x/day
Use only standardized products with verified iodine content and heavy metal testing. Total daily iodine should stay below 1100 mcg (UL).
tea
1-3 g dried thallus per cup
1-3 cups/day
Simmer 10-15 min. Monitor total iodine intake.
tincture
2-6 mL (1:5 in 25% ethanol)
TID
Less concentrated iodine than dried product; useful for digestive and topical applications.
Disclaimer: This information is largely AI-generated and reviewed by human experts at Evara Health. It is intended for educational and clinical reference purposes only and should not replace professional medical advice.
© 2026 Evara Health. All rights reserved.