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.

Evidence last reviewed: August 2026

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.

Scientific references

Effects of Daily Kelp (Laminaria japonica) Intake on Body Composition, Serum Lipid Levels, and Thyroid Hormone Levels in Healthy Japanese Adults: A Randomized, Double-Blind Study
Aoe S, Yamanaka C, Ohtoshi H — Mar Drugs, 2021
Thyroid dysfunction following a kelp-containing marketed diet
Di Matola T, Zeppa P, Gasperi M — BMJ Case Rep, 2014
The effect of Fucus vesiculosus, an edible brown seaweed, upon menstrual cycle length and hormonal status in three pre-menopausal women: a case report
Skibola CF — BMC Complement Altern Med, 2004

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