Free T4Free thyroxine · FT4
The unbound, active portion of the main thyroid hormone.
Thyroxine (T4) is the principal hormone the thyroid gland releases. Most of it circulates bound to carrier proteins and is inactive; free T4 measures the small unbound fraction available to tissues.
It matters because it is the thyroid's own output, whereas TSH measures the pituitary's instruction to the thyroid. Reading the two together separates a thyroid that is failing from a pituitary that is signalling oddly.
In practice free T4 is measured rather than total T4 because pregnancy, oral contraceptives and illness all change the amount of carrier protein, which moves the total without changing the active hormone.