A 58-yrs-old presents with increased "fullness" in her neck. Examination finds non tender diffuse enlargement of her thyroid gland. Clinically she is euthyroid. Sections from her thyroid gland reveal numerous, mainly enlarged follicles, most of which are filled with abundant colloid material. There are areas of fibrosis, hemorrhage, and cystic degeneration. No papillary structures are identified and neither colloid scalloping nor Huhle cells are present. Most likely diagnosis is?