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: Ans is A ie Serum TSH Level "A normal TSH Level excludes primary (but not secondary) hypothyrodism. If the TSH is elevated a free T4 level is needed to confirm the presence of clinical hypothyrodism, but free T4 is inferior to TSH when used as a screening test, as it will not detect subclinical or mild hypothyroidism. Circulating free T3 levels are normal in about 25% of the patients, reflecting adaptive respose to hypothyroidism. T3 measurements are therefore not indicated." - Harrison "Among the various available serum thyroid function tests, TSH is the most useful to assess gland dysfunction." ? Endocrine Surgery of Head & Neck, p 83.', 'role': 'assistant'}]