Kelp and Levothyroxine: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Levothyroxine
Kelp
No brand names on record
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Increased risk of hypo- or hyperthyroidism, altered thyroid hormone dosage requirements, and inaccurate thyroid function tests used to monitor thyroid hormone therapy
Interaction Deep Dive
Patients should be advised to avoid kelp ingestion while taking thyroid therapy7. 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 8. 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 345. Other reports describe hyperthyroidism and thyrotoxicosis secondary to iodine ingestion, in which defective autoregulation of hormone biosynthesis may have contributed 2. 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.
Why it happens (mechanism)
Iodine content of kelp may inhibit the release of thyroid hormones
Literature reports
7 reports — tap to read
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 1.
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 2.
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 3.
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 4.
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 5.
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 6.
Common questions
Can I take Kelp and Levothyroxine together?
Increased risk of hypo- or hyperthyroidism, altered thyroid hormone dosage requirements, and inaccurate thyroid function tests used to monitor thyroid hormone therapy Always confirm with your pharmacist or prescriber before making any change.
How serious is the Kelp and Levothyroxine interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Kelp or Levothyroxine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (8)
- Norman JA, Pickford CJ, Sanders TW, et al: Human intake of arsenic and iodine from seaweed-based food supplements and health foods available in the UK. Food Add Contamin 1987; 5:103-109. PubMed
- Ishizuki Y, Yamauchi K, & Miura Y: Transient thyrotoxicosis induced by Japanese Kombu. Folia Endocrinol 1989; 65:91-98. PubMed
- Tajiri J, Higashi K, Morita M, et al: Studies of hypothyroidism in patients with high iodine intake. J Clin Endocrinol Metab 1986; 63:412-417. DOI
- Shilo S & Hirsch HJ: Iodine-induced hyperthyroidism in a patient with a normal thyroid gland. Postgrad Med J 1986; 62(729):661-662. PubMed
- Yamaguchi K, Fukushima H, Uzawa H, et al: A case of iodide myxedema observed for three years under a low iodide diet-especially on the restoration of the escape mechanism from the Wolff-Chaikoff Effect. Folia Endocrinol 1984; 60:79-88.
- Okamura K, Inoue K, & Omae T: A case of Hashimoto's thyroiditis with thyroid immunological abnormality manifested after habitual ingestion of seaweed. Acta Endocrinol 1978; 88:703-712. DOI
- Anon: Kelp diets can produce myxedema in iodine sensitive individuals. JAMA 1975; 233:9-10.
- Lamberg BA: Endemic goiter-iodine deficiency disorders. Ann Med 1991; 23:367-72.
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