What is FT4?

FT4 is the thyroid gland's own output, released in response to TSH from the pituitary. Every tissue that needs thyroid hormone draws on this unbound part, so a measurement of total thyroxine would count hormone the body cannot reach.

TSH climbs when FT4 falls and eases back as FT4 recovers. TSH on its own therefore reports the pituitary's reading of FT4, not FT4 itself, and the two are tested together so that reading can be checked against the hormone.

Two things set FT4 between them: how much hormone the thyroid gland is able to make, and how hard TSH is pushing it to make more. Autoimmune disease, where the immune system attacks the gland itself, is the most common reason that capacity shifts. Thyroid hormone replacement adds to whatever the gland produces, so FT4 then reflects the dose as well as the gland.

FT4 moves as soon as gland activity or a treatment dose changes. That is why FT4 and TSH are read together in the first months after a change in gland activity or in a treatment dose, while TSH is still settling. A yearly measurement falls well outside those unsettled months, and reads what the gland is producing once its output has steadied. This is Do Health's monitoring schedule, not a general recommendation for everyone.