Download EA_QA312.3_levothyroxineseakelp

Medicines Q&As
Q&A 312.3
Can patients on levothyroxine take sea kelp?
Prepared by UK Medicines Information (UKMi) pharmacists for NHS healthcare professionals
Before using this Q&A, read the disclaimer at www.ukmi.nhs.uk/activities/medicinesQAs/default.asp
Date prepared: 29th July 2015
Background
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. The species Fucus vesiculosus,
more commonly known as bladderwrack, is reported to be commonly used in the preparation of kelp
products. Because of its iodine content kelp has traditionally been used as a source of iodine for
thyroid deficiency and as a slimming supplement. Kelp can concentrate various heavy metals, in
addition doubt over the quality of commercial seaweed preparations has been reported (1, 2).
Levothyroxine is a thyroid hormone that is mainly used as replacement therapy in the treatment of
hypothyroidism. It is also given to suppress the secretion of thyroid-stimulating hormone (TSH) in
order to treat or prevent thyroid enlargement or to treat thyroid carcinoma. The adverse effects of
levothyroxine are usually associated with excessive dosage and correspond to symptoms of
hyperthyroidism (3).
Answer
Both UK and American medicines information resources caution or advise against the concurrent use
of levothyroxine and kelp products (1, 4, 5).
Evidence of Interaction
There are no reports of an actual interaction between levothyroxine and kelp preparations in the
current literature. However, the use of kelp products has been associated with the development of
thyroid disorders, mainly hypothyroidism and hyperthyroidism. This effect has been attributed to the
iodine contained in these products (1, 2, 4-7). Several case reports about this association have been
published (1, 4).
Mechanism of Interaction
The normal daily iodine requirement ranges from 100 to 300 micrograms. Quantities of 500
micrograms to 1 mg daily probably have no untoward effects on thyroid function in most cases
(8). However, there is potential for the development of thyroid disorders if more than 150 micrograms
of iodine per day are ingested (5, 6). The average laminaria-based supplement might contain as much
as 1 mg of iodine (5). In addition the iodine content of kelp products can vary considerably, even
within products of the same brand (1, 4).
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 2 mg 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 longterm hyperthyroidism (8). These described effects of iodine on thyroid regulation can alter dosage
requirements for levothyroxine which are not predictable (4).
Conclusion
Patients should be advised to avoid kelp ingestion while taking levothyroxine therapy. Kelp products
should probably be avoided in patients suffering from thyroid disorders regardless of whether they are
taking thyroid supplementation or not as kelp can aggravate existing thyroid disorders and alter
thyroid function tests (TFTs) (4-6).
From the NHS Evidence website www.evidence.nhs.uk
1
Medicines Q&As
Summary





The term kelp can refer to a variety of brown sea weeds.
The iodine content in kelp products varies considerably, even between products of the same
brand.
Hypo- and hyperthyroidism disorders have been attributed to the use of kelp products.
UK and American resources caution or advise against the concurrent use of levothyroxine and
kelp products.
Kelp products should be avoided in patients suffering from thyroid disorders regardless of whether
they are taking thyroid supplementation.
Limitations
Documented evidence of this interaction is lacking.
References
1. Barnes J, Anderson L, Philipson J. Herbal Medicines. Fucus. Accessed 29/07/2015 via
www.medicinescomplete.com
2. Brayfield A, editor. Martindale: The Complete Drug Reference. Seaweeds, Kelps, and Wracks,
revision date 02/09/2010. Accessed 29/07/2015 via www.medicinescomplete.com
3. Brayfield A, editor. Martindale: The Complete Drug Reference. Levothyroxine Sodium. Revision
date 29/08/2013. Accessed 29/07/2015 via www.medicinescomplete.com
4. Klasco R, editor. DRUGDEX® System electronic version. Thomson Micromedex, Greenwood
Village, Colorado, USA. Levothyroxine, Drugdex evaluation. Revision date 03/07/2013. Accessed
29/07/2015 via http://www.thomsonhc.com
5. Jellin J, Gregory P, Batz F, Bonakdar R, editors. Natural Medicines Comprehensive Database.
Laminaria. Accessed 29/07/2015 via http://www.naturaldatabase.com
6. Jellin J, Gregory P, Batz F, Bonakdar R, editors. Natural Medicines Comprehensive Database.
Bladderwrack. Accessed 29/07/2015 via http://www.naturaldatabase.com
7. British Thyroid Foundation. Accessed 29/07/2015 via www.btf-thyroid.org
8. Brayfield A, editor. Martindale: The Complete Drug Reference. Iodine, revision date 23/08/2011.
Accessed 29/07/2015 via www.medicinescomplete.com
Quality Assurance
Prepared by
Sarah Cavanagh, East Anglia Medicines Information Service (based on earlier work by Kerstin
Weber)
Date Prepared
29th July 2015
Checked by
Katie Smith East Anglia Medicines Information Service
Date of check
29th July 2015
Search strategy
Embase: exp LEVOTHYROXINE AND exp KELP
exp LEVOTHYROXINE AND exp BROWN ALGAE
Medline: THYROXINE AND exp PHAEOPHYTA
MHRA website
In-house database/-resources including BNF, Martindale: the Complete Drug Reference, Herbal
Medicines, Stockley’s Drug Interactions, Natural Medicines Comprehensive Database, Meyler’s Side
Effects of Drugs, Drugdex
Internet search: MHRA, FDA, British Thyroid Foundation
From the NHS Evidence website www.evidence.nhs.uk
2