A 33-year-old woman presents with a 6-week history of fatigue, generalized aches, and difficulty concentrating. She mentions feeling low in mood and has reduced interest in her usual activities. She also complains of constipation and dry skin. Her TSH is 15 mU/L (0.27-4.2) and FT4 is 7 pmol/L (12-22). What is the most appropriate initial management?