Empeiria

Thyroxine, free

Also known as FT4, free T4, free thyroxine

C₁₅H₁₁I₄NO₄ small molecule

LOINC 3024-7 code verified serum ng/dL

What this substance is

formula
C₁₅H₁₁I₄NO₄
mass
776.872 g/mol
monoisotopic
776.6867
charge
0
InChIKey
XUIIKFGFIJCVMT-LBPRGKRZSA-N
ChEBI
CHEBI:18332

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.

n=1: 2n=2: 8n=3: 18n=4: 18n=5: 753 p⁺74 nI53 electrons · not to scale
Iodine: the element thyroid hormones need. T4 carries four iodine atoms, bonded into the molecule; this shows the free atom (ground term 2P, not spherical, so 2D only)

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. Most people don't need to get ready in any special way. If other tests use the same blood, you may need to fast for a few hours. High-dose biotin (a supplement) can give wrong results, so tell your provider if you take it.

Venous blood sample by venipuncture; free T4 is measured directly, whereas the free T4 index is calculated from total T4. Usually no preparation; fasting may be needed if other tests are ordered on the same sample. Biotin 10 mg/day interfered with specific fT4 assays, with maximal interference 2 hours after intake; suspending such biotin for 24 hours before venipuncture is recommended.

Before the test

  • Tell your provider what medicines and supplements you take, but don't stop any without talking with them.
  • If you take 10 mg a day of biotin, pause it for 24 hours before the blood draw (if your provider agrees).
  • Fast for several hours if other tests are ordered on the same sample.

Where: laboratory

Sources: Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin, Thyroid (PMC open access); Thyroxine (T4) Test, MedlinePlus (U.S. National Library of Medicine) cited · not yet checked by a person

In the lab

immunoassay

Competitive free T4 immunoassay

Free T4 from your blood competes with a tagged copy of T4 for a few antibody "seats". The fewer seats the tagged copy wins, the higher your free T4.

Routine free T4 is measured by automated single-site (competitive) immunoassay, in which free T4 competes with a labelled hormone or analogue for anti-T4 antibody binding sites under conditions meant to preserve the protein- binding equilibrium. The IFCC reference measurement procedure separates free T4 by equilibrium dialysis and quantifies it by isotope-dilution LC-MS/MS.

  • Read by: Signal from the labelled hormone/analogue bound to the antibody (inversely related to free T4)

What can throw it off

  • heterophilic antibodies
  • autoantibodies against thyroid hormones or assay components
  • variant thyroid-hormone-binding proteins (e.g. familial dysalbuminaemic hyperthyroxinaemia)
  • displacing drugs
  • high-dose biotin (streptavidin–biotin assays)
Why labs disagree: Free T4 immunoassays show high variability between assays, so results have to be interpreted against assay-specific reference intervals. In a CDC comparison, one commercial immunoassay averaged 37.7% lower than the equilibrium-dialysis LC-MS/MS reference procedure. An IFCC study suggests that aligning immunoassays to the reference procedure would allow a uniform reference interval, and CDC has started a standardisation programme, but until then each laboratory's range follows its platform. Immunoassays are also affected to differing degrees by binding-protein variation, including pregnancy.

Sources: 2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function (European Thyroid Journal, PMC open access); CDC Clinical Standardization Programs (CSP) for Free Thyroxine (FT4) to Improve the Accuracy and Reliability of FT4 Measurements (J Endocr Soc abstract, PMC open access) cited · not yet checked by a person

What it reports on

Associated symptoms

Conditions this marker features in

When high
  • high Overactive thyroid · defining
    High in overt hyperthyroidism, together with a suppressed TSH. It can be normal in milder cases.
When low
  • low Underactive thyroid · defining
    Primary hypothyroidism: TSH above and free T4 below the reference range. A high TSH with normal free T4 is the milder, subclinical form.
Either direction
  • either Mildly underactive thyroid · excludes
    Must be within the reference range. A free T4 outside it points to overt thyroid disease instead.

Ordered as part of

thyroid