Thyroxine, Free (Free T4) Unit Converter

Free T4 (free thyroxine) is the unbound, active form of thyroxine, the main hormone produced by your thyroid gland.

Most T4 in your blood (over 99%) is bound to carrier proteins like thyroxine-binding globulin (TBG), transthyretin, and albumin. Only about 0.02-0.03% circulates freely and can enter your cells to exert its effects.

Because only the free fraction is biologically active, measuring free T4 gives a more accurate picture of thyroid function than total T4, especially when protein levels are abnormal.

T4 is produced exclusively by the thyroid gland, a butterfly-shaped gland located in the front of your neck. Production is regulated by thyroid-stimulating hormone (TSH) from the pituitary gland.