A 47-year-old woman feels a 'lump' in her neck that she didn't notice 5 months before. Her physician palpates a firm nodule about 2 cm in size to the left of midline in the region of the thyroid gland. By scintigraphic scanning this nodule appears 'cold' with normal activity in the surrounding normally sized thyroid gland. Which of the following is the most likely diagnosis?