Free T3
The unbound form of T3, the most biologically active thyroid hormone.
| Typical reference range | 2.3–4.2 pg/mL |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Triiodothyronine (T3) is mostly produced by converting T4 in tissues such as the liver and kidneys. Free T3 is the unbound fraction that acts on cells to regulate metabolism, heart rate and temperature.
Why it matters
Free T3 helps detect T3-predominant hyperthyroidism and is often reviewed in people with symptoms despite normal TSH and T4. Low free T3 is common in illness and calorie restriction.
What a high result can mean
High values indicate hyperthyroidism, sometimes with normal T4 (T3 toxicosis), or use of T3-containing medication (liothyronine, desiccated thyroid).
What a low result can mean
Low values occur with hypothyroidism but also with low-calorie or low-carbohydrate diets, heavy training, chronic illness or acute stress (non-thyroidal illness syndrome), where T4 is diverted toward reverse T3.
What affects the result
Caloric restriction and illness lower free T3. Selenium and iron deficiency may impair T4-to-T3 conversion. Biotin interferes with some assays. Timing after T3-containing medication strongly affects the result.
Units and conversion
Usually reported in pg/mL.
- From pmol/L: multiply by 0.651 to get pg/mL
- From ng/dL: multiply by 10 to get pg/mL
- From pg/dL: multiply by 0.01 to get pg/mL
Also called on lab reports
T3, Free, Free Triiodothyronine, T3 Free, FT3, Triiodothyronine, Free, Triiodothyronine (T3), Free, Free T3, Serum
Related tests: Thyroid
This page is educational and does not replace advice from a healthcare professional.