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Taking excessive kelp can overload the body with iodine and cause either hypothyroidism or hyperthyroidism —conditions in which the thyroid gland either produces too little or too much thyroid hormone. 1 This is a potentially dangerous side effect and is definitely cause for caution. If your thyroid gland is already functioning incorrectly, you should avoid high doses of kelp except on a physician's advice.
Additionally, published reports describe two cases of acne apparently caused or worsened by taking large doses of kelp. 2 This effect is also believed to be due to the large amounts of iodine in the supplement.
Finally, some kelp supplements have been found to contain levels of arsenic high enough to be toxic. 3 Seawater contains highly diluted arsenic, but kelp (like other ocean life) can concentrate arsenic in its tissues, and there are reports of two people with symptoms of arsenic poisoning who had been consuming kelp.
- Konno N, Makita H, Yuri K, Iizuka N, Kawasaki K. Association between dietary iodine intake and prevalence of subclinical hypothyroidism in the coastal regions of Japan. J Clin Endocrinol Metab. 78(2):393-7.
- Harrell BL, Rudolph AH. Kelp diet: a cause of acneiform eruption [letter]. Arch Dermatol. 1976;112:560.
- Pye KG, Kelsey SM, House IM, Newland AC. Severe dyserythropoiesis and autoimmune thrombocytopenia associated with ingestion of kelp supplements. Lancet. 339(8808):1540.