A 74-year-old woman presents with pruritus vulvae, mild dry mouth, chest tightness, and precordial discomfort for over 3 months. She reports cold intolerance and constipation. Physical exam: obesity, facial and periorbital edema, dry, sparse hair and eyebrows. Heart rate 60 bpm, regular rhythm, diminished heart sounds, cardiac dullness extending 0.5 cm lateral to the left midclavicular line, liver palpable 3 fingerbreadths below the costal margin, positive hepatojugular reflux sign, and pretibial myxedema in both lower extremities. Fasting blood glucose (FBG) 8.4 mmol/L, 2-hour postprandial blood glucose (2hPPG) 13.6 mmol/L. What is the most likely initial diagnosis?