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
Serum TSH level is the most sensitive test for: 
Select and explain. 
A. Hyperthyroidism., B. Hypothyroidism., C. Both of the above., D. None.
B: Hypothyroidism.
Answer: B: Hypothyroidism. 
Explanation: None