Free T3

Measured in pg/mL

The unbound fraction of the more active thyroid hormone — useful in some specific situations, but a noisy and often unnecessary test on its own.

What it is

T3 (triiodothyronine) is the more biologically active thyroid hormone; most of it is made not by the thyroid but by other tissues converting T4 into T3, and 'free' T3 is the small unbound portion in your blood. It tends to be most informative in particular scenarios a clinician is investigating, and it's not usually the first test used to assess thyroid function — TSH and Free T4 generally carry more of the picture. Free T3 can dip during any significant illness even when the thyroid itself is fine, and results are noticeably assay-dependent, so it's easy to over-read a single value.

Typical range

  • Adults2.0–4.4 pg/mL

Source · Reference-lab convention (LabCorp) (2026)

Reference ranges vary between laboratories, and what is normal for you depends on your own history. Your own lab’s range is the one printed on your report.

Worth asking your clinician

  • In my case, does Free T3 add anything beyond my TSH and Free T4?
  • Could a recent illness or anything else have influenced this result?
  • How much do reference ranges for this test vary between labs, and does that matter for my result?