A 52-year-old woman from rural Maharashtra presents with progressive fatigue, weight gain of 8 kg over 6 months, constipation, and dry skin. She reports feeling cold even in warm weather and has noticed thinning of the outer third of her eyebrows. On examination, her heart rate is 52 bpm, blood pressure 138/88 mmHg, and skin turgor is reduced with a doughy texture. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 0.8 ng/dL (normal 0.8–1.8), total cholesterol 285 mg/dL. What is the most appropriate next step in management?