Free T4 (Thyroxine) Test

Free T4 measures the unbound, active form of thyroxine, the main hormone produced directly by the thyroid gland and the primary raw material your body converts into the more potent T3 hormone as needed.

T4 is produced in much larger quantities than T3 and serves as a stable, circulating reservoir of thyroid hormone, with a longer half-life that makes it a reliable indicator of overall thyroid output over the preceding weeks.

Measuring the 'free' fraction rather than total T4 avoids the confounding effects of protein-binding changes, such as those caused by pregnancy, liver disease, or oral contraceptive use, which can distort total hormone measurements without reflecting a genuine change in thyroid function.

Free T4 is almost always interpreted together with TSH, since the relationship between the two values — rather than either number alone — is what reveals the underlying thyroid condition.