<prompt> Investigation of choice for diagnosing primary hypothyroidism: March 2007, September 2009Decreased free T3Decreased free T4Elevated TSHAnti-TPO antibodies  <response>: Ans. C: Elevated TSH Blood TSH, free T4, free T3 and anti-TPO antibodies should be tested. Firstly measure TSH, if found elevated measure unbound T4. Elevated TSH and low free T4 ours diagnosis of primary hypothyroidism. Patients with an enlarged thyroid andor a positive anti-TPO antibody test and low free T4 with TSH 4.0 mUmL should be considered to have autoimmune hypothyroidism. Patients without an enlarged thyroid and without a positive anti-TPO antibody test but with a TSH 7.5 mUmL should also be considered to have primary hypothyroidism. Patients with a free T4 of 0.9 mg dL and a normal TSH are likely to have central hypothyroidismdrug effectssick euthyroid syndrome. Patients with symptoms of hypothyroidism but who do not meet these criterion should be watched and retested in 6 months. 