Triiodothyronine, Free (Free T3) Unit Converter

Free T3 (free triiodothyronine) is the unbound, biologically active form of the thyroid hormone T3 that circulates in your blood. Unlike bound T3, which is attached to carrier proteins, free T3 can enter cells and tissues to carry out its metabolic functions.

T3 is the active form of thyroid hormone, whereas T4 (thyroxine) is considered the inactive form. About 80% of T3 in the blood comes from the conversion of T4 to T3 in the liver and kidneys, with the remaining 20% produced directly by the thyroid gland.

Only a small fraction (less than 1%) of T3 circulates in the free form, but this is the portion that is metabolically active and available to cells.

About 20% of T3 is produced directly by the thyroid gland, while approximately 80% is converted from T4 (thyroxine) in peripheral tissues, primarily the liver and kidneys, through a process called deiodination.