Free T4 (thyroxine) is the unbound, biologically available fraction of T4 — the main hormone produced and secreted by the thyroid gland. T4 is considered a prohormone; it is largely inactive until converted to the active form T3 (triiodothyronine) in peripheral tissues, particularly the liver and kidneys.

Free T4 is used alongside TSH to assess thyroid function more completely. When TSH is abnormal, free T4 helps determine the severity and origin of the dysfunction. Low free T4 with elevated TSH confirms primary hypothyroidism, while high free T4 with low TSH confirms hyperthyroidism. Measuring free T4 rather than total T4 avoids the influence of thyroid hormone binding proteins, which can be altered by pregnancy, oestrogen therapy, or liver disease.