Some possible side effects from taking kelp, or laminaria, supplements are thyroid and kidney problems, according to WebMD. Kelp supplementation is unsafe for women who are pregnant or breastfeeding because it can affect hormones and may contain harmful chemicals.
Some kelp supplements contain very high amounts of iodine that can harm the thyroid gland, reports WebMD. The thyroid uses iodine to make hormones. Ingesting higher than normal amounts of iodine can result in overactive or underactive thyroid and can make existing thyroid problems worse. Kelp supplements can cause kidney problems by producing dangerously high potassium levels. Additionally, some kelp products contain notable amounts of the poisonous chemical element arsenic.