A 56-year-old woman 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 during swallowing. Examination reveals cachexia, temperature 37.8°C, apathetic facies, and weak voice. Skin is slightly moist. No exophthalmos. Thyroid gland is grade I enlarged without nodules or bruits. Heart rate 124/min with regular rhythm. Fine tremor (±) in both hands. Generalized muscle atrophy with muscle strength grade II-III. Which lab abnormality is most likely?