A 58-year-old woman, treated with levothyroxine 100 micrograms daily for primary hypothyroidism, attends for routine review. She feels generally well. Her recent thyroid function tests (TFTs) show: TSH 12.5 mU/L (reference range 0.4-4.0 mU/L) and Free T4 14 pmol/L (reference range 12-22 pmol/L). What is the most appropriate next step in her management?