This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thyroid, 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 and T4 level, C. Serum calcitonin assay, D. Serum TRH assay', 'role': 'user'}, {'content': 'Answer: A \nExplanation: TSH* Most informative among thyroid function test parameters because of ultra-sensitivity* Can detect subclinical hypothyroidism & subclinical hypehyroidism* Hypothyroidism - TSH |* Hypehyroidism - TSH |', 'role': 'assistant'}]