In case of hypothyroidism which investigation is most informative and most commonly used ?

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.