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Limited evidenceGeneral health

Bladderwrack

An iodine-rich brown seaweed sold for thyroid support. Its iodine content is variable and unpredictable, and the documented human effects are mainly thyroid problems rather than thyroid benefits.

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What the science says

  • Thyroid support Limited evidence

    No trial shows that bladderwrack improves thyroid function in people with normal iodine status. A randomised double-blind study of daily kelp intake in healthy Japanese adults found changes in thyroid hormone levels without health benefit, in a population whose baseline iodine intake is already high.

  • Iodine excess is the documented effect Limited evidence

    Case reports describe iodine-induced thyroid dysfunction, including thyrotoxicosis, after consumption of kelp-containing products. Because iodine content varies enormously between harvests and products, dosing is effectively unpredictable.

  • Hormonal effects in women Limited evidence

    A case report of three pre-menopausal women taking Fucus vesiculosus described lengthened menstrual cycles and altered hormone levels. Three participants and no control group make this a hypothesis, not a finding.

Dosage & safety

Studied doseNo safe or effective dose is established, because the iodine content of the raw material varies by orders of magnitude. Products typically supply 200–600 mg of dried thallus, which can mean anywhere from tens to thousands of micrograms of iodine.
SafetyCan cause both hyperthyroidism and hypothyroidism through iodine excess, and is a poor idea in any existing thyroid disease, in pregnancy, and while breastfeeding. Brown seaweeds concentrate arsenic and other heavy metals, and cases of arsenic toxicity from kelp supplements have been reported. It also has anticoagulant fucoidan content, so caution with blood thinners. If you want iodine, use a product with a stated, standardised dose.

How to take it

TimingThere is no established regimen, because the active content is not known from batch to batch.
With food?Take with food if you use it, to reduce nausea.
Worth knowingIf iodine is what you want, buy a product with a stated microgram dose — bladderwrack's iodine varies by orders of magnitude between batches, and brown seaweeds also concentrate arsenic.

Interactions

With medications

  • levothyroxine and antithyroid drugs

    Unpredictable iodine loads can swing thyroid function in either direction and destabilise treatment. do not combine without thyroid function monitoring

  • warfarin and other anticoagulants

    The fucoidan content has anticoagulant activity that may add to the drug's effect. talk to your doctor before combining

  • lithium

    Lithium and iodine both suppress thyroid hormone release, making hypothyroidism more likely. avoid seaweed supplements while on lithium

With other supplements

  • kelp, potassium iodide and other iodine sources

    Combined iodine can reach thousands of micrograms without appearing on any label. do not stack iodine-containing products

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does bladderwrack support thyroid function?

No trial shows that bladderwrack improves thyroid function in people with normal iodine status, so the evidence is limited. The documented human effects of bladderwrack are thyroid problems from iodine excess, including thyrotoxicosis in case reports.

How much iodine is in bladderwrack?

It is unpredictable: bladderwrack's iodine content varies by orders of magnitude between harvests, so typical 200–600 mg products can supply anywhere from tens to thousands of micrograms of iodine. If you want iodine, buy a product with a stated microgram dose.

Is bladderwrack safe?

Bladderwrack can cause both hyperthyroidism and hypothyroidism, and brown seaweeds concentrate arsenic, with cases of arsenic toxicity from kelp supplements reported. Avoid it with any thyroid disease, in pregnancy and while breastfeeding.

Can you take bladderwrack with levothyroxine or blood thinners?

Not without monitoring: bladderwrack's unpredictable iodine can destabilise levothyroxine or antithyroid treatment, and its fucoidan has anticoagulant activity that may add to warfarin. Avoid it entirely while on lithium.

Scientific references

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