Thyroid
Free triiodothyronineFT3
Active hormone at the receptor level; useful in suspected T3-toxicosis and conversion problems.
Sample
Serum
Clinical reference range
2.3 to 4.2 pg/mL
Adult
Conv: 2.3–4.2 pg/mL
Biology
What it measures
Mostly produced peripherally from T4 by deiodinases (D1, D2). Binds nuclear thyroid receptors with ~10× T4 affinity.
Clinical use
Why we order it
Diagnose T3-predominant hyperthyroidism; evaluate non-thyroidal illness syndrome (low T3 with normal TSH).
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