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
In case of hypothyroidism which investigation is most informative and most commonly used: 
Select and explain. 
A. Serum TSH level, B. Serum T3 and T4 level, C. Serum calcitonin assay, D. Serum TRH assay
A: Serum TSH level
Answer: A: Serum TSH level 
Explanation: TSH* Most informative among thyroid function test parameters because of ultra-sensitivity* Can detect subclinical hypothyroidism & subclinical hypehyroidism* Hypothyroidism - TSH |* Hypehyroidism - TSH |