Free T3 (Free Triiodothyronine)

What Free T3 Measures

Free T3 measures the unbound share of triiodothyronine in blood. Almost all circulating thyroid hormone rides on carrier proteins such as thyroxine-binding globulin, and only a small fraction travels free.^[1] That free fraction is the part that can enter cells and act, which is why labs report it separately from total T3. Our free T4 entry covers the total-versus-free distinction, and the same logic applies here.

T3 is the active form of thyroid hormone. Thyroid receptors bind T3 far more strongly than T4, which leaves T4 acting mostly as a reservoir.^[1] Inside the cell nucleus, T3 binds receptors called TRα and TRβ that switch target genes on and off. Through that gene-level control, thyroid hormone influences energy expenditure, heart rate, and lipid metabolism.^[2]

The thyroid gland itself secretes mostly T4 and only a small share of T3.^[1] Deiodinase enzymes in peripheral tissues then remove one iodine atom from T4 to make T3, and research on these enzymes shows they supply much of the T3 found in plasma.^[3] The same enzyme family can remove a different iodine atom instead, which produces reverse T3, an inactive byproduct.^[1]

How It's Measured

Free T3 comes from a standard venous blood draw, and labs usually measure it with an immunoassay.^[4] On a thyroid panel it sits alongside TSH and free T4. Clinical guidance treats TSH as the usual first marker, with the free hormones measured in specific follow-on situations rather than by default.^[5] US labs typically report free T3 in pg/mL, while many other countries use pmol/L.

Why People Track Free T3

Free T3 appears on the expanded thyroid panels that many at-home hormone tests and full blood workups now include. Each of the three common thyroid markers tells a different part of one story. TSH is the pituitary's signal to the thyroid gland, free T4 is the main hormone the gland releases, and free T3 is the active hormone that tissues respond to.^[1] Anyone who tests more than once ends up with a series of results worth keeping in one place.

For the app-and-wearable side of thyroid-adjacent tracking, see our explainer on tracking thyroid symptoms alongside your cycle.

How Go Go Gaia Tracks Free T3

Free T3 is a lab value, so it enters Go Go Gaia through lab upload or manual entry rather than a wearable. Upload a lab report or type in a result and the app charts it over time next to your cycle days, symptoms, and other lab values, in pg/mL with automatic conversion from pmol/L. Repeat tests build into one timeline instead of a stack of PDFs.

What the Research Shows

Three deiodinase enzymes, called D1, D2, and D3, coordinate thyroid hormone activation and inactivation across tissues, and research highlights D2 as a significant contributor to the T3 found in human plasma.^[3]

T3's effects run through nuclear receptors encoded by two genes, TRα and TRβ, which are expressed differently across tissues. That receptor distribution helps explain why thyroid hormone touches so many systems at once, from heart rate to energy expenditure to thermogenesis in brown fat.^[2]

Measurement itself is an active research topic. Reviews of free thyroid hormone testing note that the immunoassays most labs use can be affected by changes in serum binding proteins, so assay method is part of the picture when comparing results.^[4]

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Where does T3 come from?

The thyroid gland makes some T3 directly, but its output is mostly T4. Deiodinase enzymes in tissues such as the liver, kidney, and muscle then remove one iodine atom from T4 to produce T3, and that peripheral conversion supplies most of the T3 in blood.

Is free T3 the same as total T3?

No. Total T3 counts all the T3 in blood, including the large share bound to carrier proteins. Free T3 counts only the small unbound fraction, which is the part that can enter cells and reach receptors. The same total-versus-free distinction applies to T4.

How is free T3 measured?

Free T3 is measured from a standard blood draw, usually with an immunoassay. It often appears on thyroid panels alongside TSH and free T4. US labs typically report it in pg/mL, while many other countries use pmol/L.