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
In case of hypothyroidism which investigation is most informative and most commonly used - 
Select and explain. 
A. Serum TSH Level, B. Serum T3, T4 Level, C. Serum Calcitonin assay, D. Serum TRH assay
A: Serum TSH Level
Answer: A: Serum TSH Level 
Explanation: None