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) hypothyroidism. If the TSH is elevated a free T4 level is needed to confirm the presence of clinical hypothyroidism, 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 response 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.