What it means
Most thyroxine (T4) in the blood is bound to proteins; the small free part is the active hormone that reaches the cells. Free T4 is read with the TSH: a high TSH with a low free T4 points to hypothyroidismhy-poh-THY-roy-diz-umA condition in which the thyroid gland makes too little thyroid hormone.hypo- below, underthyr/o thyroid gland-ism conditionFull entry , and a low TSH with a high free T4 points to hyperthyroidismhy-per-THY-roy-diz-umA condition in which the thyroid gland makes too much thyroid hormone.hyper- above, overthyr/o thyroid gland-ism conditionFull entry . It is also used to adjust thyroid hormone doses.
For the nurse
Note thyroid medicines and the time of the last dose. Biotin supplements can upset some thyroid tests, so ask about them.
At a glance
| Normal range | About 0.8–1.8 ng/dL [10–23 pmol/L]; ranges vary by lab. |
|---|---|
| Also written | FT4, free thyroxine |
Related terms
hypothyroidism
A condition in which the thyroid gland makes too little thyroid hormone.
hyperthyroidism
A condition in which the thyroid gland makes too much thyroid hormone.
thyr/o
A word part that means the thyroid gland.