A 52-year-old woman from rural Maharashtra presents with progressive fatigue, weight gain of 8 kg over 6 months, and constipation. She reports cold intolerance and has noticed thinning of eyebrows. On examination, her pulse is 52/min, blood pressure 138/88 mmHg, skin is dry and pale, and there is a firm, non-tender thyroid enlargement. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 0.8 ng/dL (normal 0.8–1.8), and anti-TPO antibodies 320 IU/mL (normal <35). What is the most likely diagnosis?