A 38-year-old woman has had a feeling of fullness in her neck for the past year. She is otherwise asymptomatic. Her physician's assistant palpates an enlarged but nontender thyroid gland. She has no difficulty swallowing. There is no palpable lymphadenopathy. She has no fever. Her serum TSH is 3.5 mU/L (normal) with total thyroxine of 8.2 micrograms/dL (normal). Thyroid peroxidase antibody is not detected. Two years later, her thyroid has not appreciably changed in size. Which of the following conditions is she most likely to have?