Hashimoto's thyroiditis is an autoimmune thyroiditis where autoantibodies within the body cause the destruction of thyroid follicular cells, leading to thyroid cell failure. During the course of Hashimoto's thyroiditis, hyperthyroidism may occur. Due to the destruction of thyroid follicles by autoantibodies, a significant release of thyroid hormones into the blood occurs, resulting in a hyperthyroid state. Therefore, symptoms such as weight loss, heat intolerance, and excessive sweating which are typical of hyperthyroidism may also appear in Hashimoto's thyroiditis. However, as the disease progresses, the condition gradually leads to decreased thyroid function.