<prompt> In case of hypothyroidism which investigation is most informative and most commonly used:Serum TSH levelSerum T3 and T4 levelSerum calcitonin assaySerum TRH assay  <response>: TSH Most informative among thyroid function test parameters because of ultra-sensitivity Can detect subclinical hypothyroidism subclinical hypehyroidism Hypothyroidism - TSH Hypehyroidism - TSH 