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: Medicine
The best marker to diagnose thyroid related disorder is 
Select and explain. 
A. T3, B. T4, C. TSH, D. Thyroglobulin
C: TSH
Answer: C: TSH 
Explanation: Answer is C (TSH) : The enhanced sensitivity and specificity of TSH assays have greatly improved laboratory assessment of thyroid function - Harrison With rare exceptions a normal TSH level excludes a primary abnormality of thyroid function.- Harrison Approach to thyroid testing: Because TSH change dramatically in response to alteration of free T4 and T3, a logical approach to thyroid testing is to determine first whether TSH is suppressed, normal or elevated - Harrison