A 55-year-old woman came to a doctor with complaints of thyroid enlargement that could be observed throughout the last 2 years and a discomfort during swallowing. Objectively, she has signs of hypothyroidism. The thyroid gland on palpation is dense, non-fused with the surrounding tissues and mobile during swallowing. The regional lymph nodes are not enlarged. Laboratory testing detects antithyroid antibodies in the blood serum. What is the most likely diagnosis in this case?