Free T4 (free thyroxine)
The active, unbound form of the thyroid's main hormone - a key marker of thyroid output.
T4 (thyroxine) is the main hormone produced by the thyroid gland. Most T4 in the blood is bound to carrier proteins and cannot enter cells. The small fraction that is unbound - called free T4 (fT4) - is the biologically active form that tissues can actually use.
Free T4 is the standard way to measure thyroid hormone output in blood tests, usually alongside TSH. A low free T4 with a raised TSH points to hypothyroidism; a high free T4 with a suppressed TSH is consistent with hyperthyroidism.
T4 acts in the body largely as a prohormone - cells convert it into the more potent T3 (triiodothyronine) as needed. People taking levothyroxine are replacing the thyroid's T4 production; their cells then convert it to T3.
The reference range for free T4 varies between laboratories and assays used. Results should always be interpreted alongside TSH and in the context of symptoms and treatment history.
Common questions
Why is free T4 measured rather than total T4?
Related terms
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