A 40-year-old female patient complains of having a bulge on the anterior surface of neck for 5 years. Objectively: Ps- 72/min., arterial pressure - 110/70 mm Hg, in the right lobe of thyroid gland palpation reveals a mobile node 4x2 cm in size, the left lobe is not palpable, the basal metabolic rate is 6%. What is the most likely diagnosis?