Hashimoto's thyroiditis is a type of autoimmune thyroiditis, which can present in various phases regarding thyroid function, manifesting as hyperthyroidism, hypothyroidism, or normal thyroid function. Therefore, for patients with Hashimoto's thyroiditis, it is necessary to check thyroid function to understand its status. Since Hashimoto's thyroiditis is an autoimmune thyroiditis, the thyroid peroxidase antibodies and thyroglobulin antibodies are generally elevated. Additionally, it is necessary to perform thyroid ultrasound to assess the thyroid's condition. For some difficult-to-distinguish patients, a further examination such as a 131I uptake test can be conducted, where the uptake rate in Hashimoto's thyroiditis is reduced. Even further, a fine needle aspiration biopsy of the thyroid can be performed, where we can observe significant lymphocyte infiltration.