The severity of hypothyroidism varies. Some patients have no clinical symptoms and normal T3 and T4 levels, but only elevated TSH levels (>4.0 μU/L) are subclinical hypothyroidism; spontaneous subclinical hypothyroidism is more common in the population, and the prevalence of subclinical hypothyroidism is 1% to 10% worldwide. Subclinical hypothyroidism does not require medication when the TSH is less than 10 μU/L. Eugenol replacement therapy should be started when the TSH is greater than or equal to 10 μU/L. Iodine is an important raw material necessary for the synthesis of thyroid hormones. The general requirement of iodine is 120-165 μg for adult men and 100-115 μg for adult women. Kelp and seafood are the main sources of iodine, and some iodine is also found in green leafy vegetables, meat, eggs, dairy, grains and salt with added iodine. If you have hypothyroidism, you can consume seafood in moderation, but never in excess. For hypothyroidism caused by surgery such as thyroid nodules, you can eat more seafood to supplement the raw materials necessary to synthesize thyroid hormones. For hypothyroidism caused by Hashimoto’s thyroiditis, in principle, you can consume seafood, but excessive intake is not recommended because excessive iodine intake can elevate thyroid autoantibodies, which can aggravate thyroiditis. In addition, patients with hyperthyroidism should not consume foods that contain a lot of iodine such as seafood like kelp, nori and seafood. For hypothyroidism caused by isotope treatment of hyperthyroidism, strict avoidance of iodine is recommended as long as the thyroid hormone receptor antibodies are positive. This is because the remaining part of the thyroid follicle, except for the part destroyed by isotope, is still taking in iodine, which may lead to a recurrence of hyperthyroidism.