<prompt> In case of hypothyroidism which investigation is most informative and most commonly used ?Serum TSH LevelSerum T3, T4 LevelSerum Calcitonin assaySerum TRH assay  <response>: Ans is A ie Serum TSH Level A normal TSH Level excludes primary but not secondary hypothyrodism. If the TSH is elevated a free T4 level is needed to confirm the presence of clinical hypothyrodism, but free T4 is inferior to TSH when used as a screening test, as it will not detect subclinical or mild hypothyroidism. Circulating free T3 levels are normal in about 25 of the patients, reflecting adaptive respose to hypothyroidism. T3 measurements are therefore not indicated. - Harrison Among the various available serum thyroid function tests, TSH is the most useful to assess gland dysfunction. ? Endocrine Surgery of Head Neck, p 83. 