Free T3
What it measures, and what changes the number.
Triiodothyronine (T3) is the most active thyroid hormone. Most T3 in the blood is bound to carrier proteins. The free T3 test measures the small unbound fraction that is available to tissues.
Free T3 is usually read alongside TSH and free T4. Together these show how the pituitary signal and the thyroid output relate to each other. Total T3, reverse T3 and thyroid antibodies are sometimes added for a fuller picture.
Most labs measure free T3 by immunoassay, and results differ between assay platforms. High intake of biotin can interfere with some immunoassays. Levels vary somewhat with time of day and acute illness, so many people have the sample drawn at a consistent time. Reports may list it as T3, FREE or Triyodotironina Libre (T3L), in pmol/L or pg/mL.
Usually reported in pmol/L or pg/mL.
Often read alongside: TSH, Free T4, Total T3, Reverse T3, Anti-TPO antibodies
Calculators that use it: Free T3 to free T4 ratio