A 56-year-old female presents with unintentional weight loss over 3 months. For the past month, she has had diarrhea 3-4 times daily without significant abdominal pain, unresponsive to antibiotics. Over the last week, she developed extreme fatigue, lethargy, inability to stand, and choking cough while swallowing. Examination reveals cachexia, temperature 37.8°C, apathy, and weak voice. Skin is slightly moist. No exophthalmos. Thyroid is grade I enlarged without nodules or bruit. Heart rate 124/min with regular rhythm. Fine tremor in both hands (±). Generalized muscle atrophy with muscle strength II-III. Which test should be prioritized for initial diagnosis?