This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'In case of hypothyroidism which investigation is most informative and most commonly used - \nSelect and explain. \nA. Serum TSH Level, B. Serum T3, T4 Level, C. Serum Calcitonin assay, D. Serum TRH assay', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]