<prompt> Which of the following is the most useful investigation for thyroid function: September 2011T3T4TSHTRH  <response>: 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 