Using kelp in patients on levothyroxine or with thyroid disorders

Tim Meadows, Medicines Information Pharmacist, Midlands and East Medicines Advice Service (East site)Published

Information on why patients use kelp, how it can affect thyroid function and how this can affect levothyroxine effectiveness.

Why patients take kelp

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.

Iodine and kelp

Normal daily iodine requirement

The normal daily iodine requirement ranges from 100micrograms to 300micrograms, with the UK reference nutrient intake value of 140micrograms for adults.

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.

Kelp and the thyroid

Kelp’s effect on thyroid regulation

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. This 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. These described effects of iodine on thyroid regulation can alter dosage requirements for levothyroxine which are not predictable.

Avoiding kelp in patients with thyroid disorders

Kelp can aggravate existing thyroid disorders.  Kelp products should be avoided in patients suffering from thyroid disorders regardless of whether they are taking thyroid supplementation or not.

Avoiding kelp in patients taking levothyroxine

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. The use of kelp has been associated with the development of thyroid disorders, mainly hypothyroidism and hyperthyroidism, attributed to the iodine contained in these products. Several case reports about this association have been published.

Kelp’s effect on thyroid function tests

Kelp can aggravate existing thyroid disorders presented by an alteration in thyroid function tests.