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
Which of the following is the most useful investigation for thyroid function: September 2011 
Select and explain. 
A. T3, B. T4, C. TSH, D. TRH
C: TSH
Answer: C: TSH 
Explanation: Ans. C: TSH TSH is usually regarded as the most useful investigation of thyroid function Thyroid pathology: Primary hypehyroidism: - Decreased TSH - Raised T4 Secondary hypehyroidism: Raised TSH Hypothyroidism: Raised TSH