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': 'Which of the following is the most useful investigation for thyroid function: September 2011 \nSelect and explain. \nA. T3, B. T4, C. TSH, D. TRH', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 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', 'role': 'assistant'}]