Levothyroxine with Kelp Interaction Details
Brand Names Associated with Levothyroxine
- Levo-T®
- Levothroid®
- Levothyroxine
- Levoxyl®
- Synthroid®
- Tirosint®
- Unithroid®
Medical Content Editor Dr. Brian Staiger, PharmD
Last updated
Nov 05, 2023
Interaction Effect
Increased risk of hypo- or hyperthyroidism, altered thyroid hormone dosage requirements, and inaccurate thyroid function tests used to monitor thyroid hormone therapy
Interaction Summary
Patients should be advised to avoid kelp ingestion while taking thyroid therapy. Endemic goiter has been reported in communities where seaweed is used as a staple food. The major endemic areas are in Asia, Africa, and Latin America . It would appear that the iodine content of kelp products varies considerably within products and the effect of the iodine on the thyroid gland is variable. The predominant effect of kelp seems to be inhibition of release of thyroid hormones, although reports of hyperthyroidism have also been reported. Several cases of hypothyroidism have been reported which were reversed upon discontinuation of seaweed (kelp) use . Other reports describe hyperthyroidism and thyrotoxicosis secondary to iodine ingestion, in which defective autoregulation of hormone biosynthesis may have contributed . These variable effects may alter dosage requirements and affect monitoring parameters (thyroid function tests). Avoidance of iodine-containing products such as kelp when taking thyroid therapy is recommended.
Severity
Moderate
Onset
Delayed
Evidence
Probable
How To Manage Interaction
Most people can eat kelp without harmful effects. Ingestion of kelp may alter thyroid treatment in patients with thyroid disorders.
Mechanism Of Interaction
Iodine content of kelp may inhibit the release of thyroid hormones
Literature Reports
A) The concentration of iodine in 21 different kelp products ranged from 100 micrograms/gram (mcg/gram) to 4200 mcg/gram. While the majority of the products originated in the United Kingdom, the product containing 4200 mcg/gram was produced in Australia. Products within the same brand but purchased at different stores contained varying amounts of iodine with differences as much as 100 mcg. An unacceptable level of variance was present in these products .
B) The typical iodine content of kelp is listed as 0.15 milligram (mg) per tablet. The effect of iodide on hyperthyroidism occurs over a variable period of time and the beneficial effect may disappear (Farwell & Braverman, 1996).
C) Transient thyrotoxicosis induced by Kombu (kelp) was reported in two Japanese women, 42 and 59 years old, who had ingested 28 to 150 mg of iodine daily in their diet for 1 month and 1 year, respectively. Their serum organic iodine levels were 2.05 +/- 0.99 mcg/dL. Within 1 month of Kombu discontinuation, thyroid function tests normalized .
D) Two cases of reversible hypothyroidism were reported in patients ingesting seaweed (kelp) daily containing 25.4 mg and 43.1 mg iodine, along with 2 cases of irreversible hypothyroidism in patients ingesting seaweed daily containing 15 mg and 20 mg iodine. In cases of reversible hypothyroidism, restricting dietary intake of seaweed resulted in normalization of thyroid function .
E) A 72-year-old female developed hyperthyroidism after ingesting 4 to 6 daily tablets of sea-kelp for one year (Vitalia(R); Norway), each containing 0.7 mg of iodine. She experienced complete suppression of TSH (after TRH challenge) and thyroid antibodies were undetectable. Within 6 months of discontinuation of the sea-kelp tablets, thyroid hormone levels and TSH response to TRH administration normalized from admission values (T3 was 185.3 nanomole/liter (nmol/L) (normal: 57.9-160.9 nmol/L) and T4 was 4.38 nmol/L (normal: 1.07-3.38 nmol/L)) to 108.1 nmol/L and 2.15 nmol/L, respectively. The authors postulated that defective autoregulation of hormone biosynthesis may have contributed to this patients' presentation .
F) A 47-year-old woman was hospitalized with symptoms suggestive of hypothyroidism after ingestion of 50 grams of seaweed (Kombu) for 5 years and at least 1 gram of Wakame for 6 months. Upon admission, her serum thyroxine (T4) was 1.3 mcg/deciliter (dL), serum triiodothyronine (T3) was 47 nanograms (ng)/dL, and thyroid stimulating hormone (TSH) was 132 microunits (microU)/mL. When seaweed was discontinued from her diet for 2 weeks, T4 rose to 6.3 mcg/dL, T3 rose to 133 ng/dL, and TSH decreased to 11 microU/mL. After 3 years of a seaweed-free diet, the patient remained euthyroid .
G) Goitrous hypothyroidism was reported in a 44 year-old man who ingested "large amounts of seaweed" (amount unspecified). He had a positive test for antithyroid microsomal antibody suggestive of thyroid autoimmune processes. Within 3 months of iodide restriction, serum T4 level and TSH level normalized .
Levothyroxine Overview
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Levothyroxine is used to treat hypothyroidism (condition where the thyroid gland does not produce enough thyroid hormone). It is also used with surgery and radioactive iodine therapy to treat thyroid cancer. Levothyroxine is in a class of medications called hormones. It works by replacing thyroid hormone that is normally produced by the body.
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Without thyroid hormone, your body cannot function properly, which may result in poor growth, slow speech, lack of energy, excessive tiredness, constipation, weight gain, hair loss, dry, thick skin, increased sensitivity to cold, joint and muscle pain, heavy or irregular menstrual periods, and depression. When taken correctly, levothyroxine reverses these symptoms.
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Definitions
Severity Categories
Contraindicated
These drugs, generally, should not be used together simultaneously due to the high risk of severe adverse effects. Combining these medications can lead to dangerous health outcomes and should be strictly avoided unless otherwise instructed by your provider.
Major
This interaction could result in very serious and potentially life-threatening consequences. If you are taking this drug combination, it is very important to be under close medical supervision to minimize severe side effects and ensure your safety. It may be necessary to change a medication or dosage to prevent harm.
Moderate
This interaction has the potential to worsen your medical condition or alter the effectiveness of your treatment. It's important that you are monitored closely and you potentially may need to make adjustments in your treatment plan or drug dosage to maintain optimal health.
Minor
While this interaction is unlikely to cause significant problems, it could intensify side effects or reduce the effectiveness of one or both medications. Monitoring for changes in symptoms and your condition is recommended, and adjustments may be made if needed to manage any increased or more pronounced side effects.
Onset
Rapid: Onset of drug interaction typically occurs within 24 hours of co-administration.
Delayed: Onset of drug interaction typically occurs more than 24 hours after co-administration.
Evidence
Level of documentation of the interaction.
Established: The interaction is documented and substantiated in peer-reviewed medical literature.
Theoretical: This interaction is not fully supported by current medical evidence or well-documented sources, but it is based on known drug mechanisms, drug effects, and other relevant information.
How To Manage The Interaction
Provides a detailed discussion on how patients and clinicians can approach the identified drug interaction as well as offers guidance on what to expect and strategies to potentially mitigate the effects of the interaction. This may include recommendations on adjusting medication dosages, altering the timing of drug administration, or closely monitoring for specific symptoms.
It's important to note that all medical situations are unique, and management approaches should be tailored to individual circumstances. Patients should always consult their healthcare provider for personalized advice and guidance on managing drug interactions effectively.
Mechanism Of Interaction
The theorized or clinically determined reason (i.e., mechanism) why the drug-drug interaction occurs.
Disclaimer: The information provided on this page is for informational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional regarding your specific circumstances and medical conditions.
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Information for our drug interactions is compiled from several drug compendia, including:
The prescribing information for each drug, as published on DailyMED, is also used.
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