Hashimoto thyroiditis: A progressive disease of the thyroid gland characterized by the presence of antibodies directed against the thyroid, and by infiltration of the thyroid gland by lymphocytes. Hashimoto thyroiditis is the most common cause of hypothyroidism in North America and Europe. In this condition, the thyroid gland is usually enlarged (goiter) and has a decreased ability to make thyroid hormones. Hashimoto disease predominantly affects women, and can be inherited. It is also known as autoimmune thyroiditis and Hashimoto disease.