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Herb library · Seaweed · Laminaria species (order Laminariales), e.g. Laminaria hyperborea, L. japonica

Kelp

Large brown seaweeds eaten as food and sold as tablets for minerals, thyroid, weight and hair. Kelp is one of the richest food sources of iodine, and kelp supplements have caused thyroid problems and, in one case, arsenic poisoning.

Serious warning

Kelp supplements can deliver far more iodine than the body can safely handle. PubMed case reports describe a woman with no thyroid disease who developed hyperthyroidism after 3 months of kelp tablets, and a woman with hair loss, memory loss and vomiting traced to arsenic in her kelp supplements; eight of nine kelp products tested in that report contained arsenic above FDA tolerance levels for some foods.

Traditional use

Kelp has been used in several traditional medicine systems and as a food. Over-the-counter kelp products are promoted for wellness, hair and nail growth, blood sugar, blood pressure, migraine, indigestion and constipation. It is also used in toothpaste and ice cream (as alginate) and as fertilizer.

What studies show

  • Limited

    Weight: in a 2021 Japanese trial, 50 overweight adults took iodine-reduced kelp powder (6 grams a day) or placebo for 8 weeks. Body fat percentage fell in men but not in women, and thyroid hormones did not rise. The powder was specially processed to lower its iodine.

  • Insufficient

    Hair, blood pressure, blood sugar and other promoted uses: LiverTox notes kelp is not approved for any condition, and no large trials have reported on it.

Be careful of

Cautions

  • AmountThe iodine in seaweed varies enormously, from 16 to 2,984 mcg per gram in commercial products (ODS). The adult upper limit for iodine is 1,100 mcg a day; one gram of some seaweeds exceeds it.
  • ConditionToo much iodine can cause goiter, an underactive or overactive thyroid, and thyroid inflammation. People with autoimmune thyroid disease can react to intakes that are safe for others.
  • InteractionHigh iodine intake with antithyroid drugs such as methimazole can cause hypothyroidism. Kelp can also take up metals such as arsenic, mercury, lead and cadmium from the water it grows in.
  • PregnancyLiverTox notes kelp was once used in attempts to induce abortion. Iodine needs rise in pregnancy, but the upper limit still applies; ask your clinician before taking kelp while pregnant or breastfeeding.
  • Ask a clinicianTalk with a clinician or pharmacist before using this if you take prescription medicines, are pregnant or breastfeeding, or are giving it to a child.

Sources

  1. Kelp (LiverTox) (opens in a new tab) · NIH · read September 25, 2026
  2. Iodine: Fact Sheet for Health Professionals (opens in a new tab) · NIH ODS · read September 25, 2026
  3. Effects of Daily Kelp (Laminaria japonica) Intake on Body Composition, Serum Lipid Levels, and Thyroid Hormone Levels (2021) (opens in a new tab) · NIH · read September 25, 2026
  4. Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed (case report, 2019) (opens in a new tab) · NIH · read September 25, 2026
  5. Case report: potential arsenic toxicosis secondary to herbal kelp supplement (2007) (opens in a new tab) · NIH · read September 25, 2026

Reviewed September 25, 2026.