A 48-year-old woman presents with fatigue, weight gain, constipation, and feeling cold over the past few months. Her TSH level is 15 mU/L (Ref range 0.4-4.0 mU/L) and her Free T4 level is 7 pmol/L (Ref range 12-22 pmol/L). What is the most appropriate initial management according to NICE NG145?