Thyroid
Free thyroxineFT4
Active fraction of T4; the prohormone of T3.
Sample
Serum
Clinical reference range
0.8 to 1.8 ng/dL
Adult
Conv: 0.8–1.8 ng/dLFunc: 1.1–1.5 ng/dL
Biology
What it measures
T4 is bound 99.97% to TBG, transthyretin, and albumin. Free T4 is the unbound, active fraction.
Clinical use
Why we order it
Confirm thyroid status; mandatory in pregnancy and central hypothyroidism workup.
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