Sargassum, a type of brown seaweed, has a long history of use in traditional East Asian medicine, particularly in Traditional Chinese Medicine (TCM), for supporting the thyroid gland and treating conditions such as goiter (an enlarged thyroid). The rationale behind this use is primarily due to Sargassum's naturally high iodine content, as iodine is an essential nutrient required for the synthesis of thyroid hormones. In TCM, Sargassum (commonly referred to as "Hai Zao") is classified as a herb that "softens hardness" and is often included in formulas for thyroid swelling and related disorders.

While the traditional use of Sargassum for thyroid health is well-documented, scientific validation is limited. Iodine supplementation is a well-established treatment for certain types of thyroid dysfunction, such as iodine-deficiency goiter. However, the specific efficacy and safety of Sargassum itself have not been extensively studied in modern clinical trials. Concerns exist regarding variable iodine content in seaweeds and the potential presence of heavy metals or other contaminants. Additionally, excessive iodine intake can sometimes exacerbate thyroid disorders.

In summary, Sargassum's use for supporting the thyroid is justified primarily by historical and traditional practices rather than robust modern scientific evidence. Its iodine content provides a plausible mechanism for its effects, but more research is needed to confirm its efficacy and safety for thyroid health in contemporary settings.