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Kelp and hyperthyroidism (thyrotoxicosis)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Kelp is a generic term that refers to Laminaria and Macrocystis species of brown seaweeds, although in practice the term is often used in reference to species of Fucus. Because of its iodine content kelp has traditionally been used as a source of iodine for thyroid deficiency and as a slimming supplement (1)

  • kelp is a seaweed preparation - it is often recommended by alternative practitioners for patients with presumed thyroid dysfunction (2)
  • kelp provides a high iodine load to the thyroid gland - this may lead to hyper or hypothyroidism

Iodine content of kelp and its effect

The iodine content of kelp products can vary considerably, even within products of the same brand. The average laminaria-based supplement might contain as much as 1mg of iodine. Quantities of 500micrograms to 1mg daily probably have no untoward effects on thyroid function in most cases. However, there is potential for the development of thyroid disorders if more than 150micrograms of iodine per day are ingested.

  • normal daily iodine requirement ranges from 100micrograms to 300micrograms, with the UK reference nutrient intake value of 140micrograms for adults (1)

Kelp and Thyroid Function (1):

  • when progressively larger doses of iodine are ingested there is an initial rise in thyroid hormone production, but as doses continue to rise production is reduced
    • effect is usually seen with doses of more than about 2mg daily, but is normally transient, and adaptation can occur on repeated dosage
    • if individuals do not adapt to receiving large doses of iodine, chronic inhibition of thyroid hormone synthesis occurs leading to goitre and hypothyroidism.
    • excess iodine may also induce long-term hyperthyroidism
    • effects of iodine on thyroid regulation can alter dosage requirements for levothyroxine which are not predictable
      • patients should be advised to avoid kelp ingestion while taking levothyroxine therapy. Although UK and American information resources caution or advise against taking levothyroxine with kelp, there are no published reports of an interaction

Kelp can aggravate existing thyroid disorders (1)

  • Kelp products should be avoided in patients suffering from thyroid disorders regardless of whether they are taking thyroid supplementation or not
  • Kelp can aggravate existing thyroid disorders presented by an alteration in thyroid function tests


  1. NHS Specialist Pharmacy Service (May 24th 2021). Using kelp in patients on levothyroxine or with thyroid disorders
  2. Prescriber (2002), 13 (10), 50-68.

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