EmpeiriaTriiodothyronine, free
Also known as
FT3, free T3
C₁₅H₁₂I₃NO₄ small molecule
What this substance is
- formula
- C₁₅H₁₂I₃NO₄
- mass
- 650.976 g/mol
- monoisotopic
- 650.79005
- charge
- 0
- InChIKey
- AUYYCJSJGJYCDS-LBPRGKRZSA-N
- ChEBI
- CHEBI:18258
Drawn from ChEBI's own curated 2D drawing, accepted only after its InChIKey, formula and charge matched the ChEBI release. Nothing here is hand-entered. Two masses are given because they answer different questions: the average is natural-abundance weighted and drives unit conversion; the monoisotopic value uses only the commonest isotope of each element and is what mass spectrometry measures.
Each molecule is drawn from ChEBI's curated 2D drawing, accepted only after its InChIKey, formula and charge matched the ChEBI release. Atom diagrams are checked against the NIST Atomic Spectra Database.
3D model
Generated by RDKit from ChEBI's verified structure, and accepted only because its own 3D geometry reproduces ChEBI's InChIKey. A flexible molecule takes many shapes; this is one low-energy one. Drag to rotate, scroll to zoom.
Reference ranges
Checking your own result against this range needs the running application; this is a static copy. Nothing you type anywhere in Empeiria is ever sent to a server.
This open edition shows a range only where a public-domain or openly licensed source states one, and none does for this marker yet. Compare your result with the range printed on your own lab report.
How it is measured
In practice
A health worker takes a small blood sample from a vein in your arm with a needle. You don't need to do anything special to get ready. Some medicines and supplements can change the result. Don't stop any medicine unless your provider tells you to.
Venous blood sample by venipuncture. A free T3 test measures only unbound T3, whereas a total T3 test measures bound and free T3 together. No special preparation is required; some medicines and supplements can affect results, and the provider will advise whether any should be stopped.
Before the test
- Don't stop any medicine unless your provider tells you to.
Where: laboratory
Source: Triiodothyronine (T3) Tests, MedlinePlus (U.S. National Library of Medicine) cited · not yet checked by a person
In the lab
Competitive free T3 immunoassay
The free T3 in your blood races a man-made, tagged copy of T3 to grab onto a small number of antibodies. The less tagged copy that gets caught, the more free T3 you have.
Most laboratories measure free T3 with an automated single-site (competitive) immunoassay, in which free T3 in the sample competes with a labelled hormone or hormone analogue for binding sites on an anti-T3 antibody.
- Read by: Signal from the labelled hormone/analogue bound to the antibody (inversely related to free T3)
What can throw it off
- heterophilic antibodies
- autoantibodies against thyroid hormones or assay components
- variant thyroid-hormone-binding proteins
- displacing drugs
- high-dose biotin (streptavidin–biotin assays)
Sources: 2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function (European Thyroid Journal, PMC open access); Soldin OP, Soldin SJ. Thyroid hormone testing by tandem mass spectrometry. Clin Biochem (PMC open access) cited · not yet checked by a person
What it reports on
-
thyroid gland
secreted by
The thyroid gland makes T3 and T4, and a T3 test is used to check whether it is making too much hormone.
-
liver
synthesized by
Most T3 is made outside the thyroid by converting T4, and the liver enzyme DIO1 has long been thought to play a major part.
Associated symptoms
Conditions this marker features in
-
high
Overactive thyroid
· defining
High in overt hyperthyroidism. In milder cases free T3 may be the only raised hormone, so it is measured when T4 is normal.
- low Underactive thyroid · supportive