Free T3
Free T3 is the active form of thyroid hormone, which influences metabolism, energy use and body temperature.
Free T3 (triiodothyronine) is the active form of thyroid hormone. It influences metabolism, how the body uses energy and body temperature. “Free” means the part that is not bound to proteins in the blood and can act in the tissues. Most T3 is made from T4 in the body’s tissues. Retesting shows how it changes over time.
Long periods of strict dieting or low energy intake, a recent illness, stress and very hard training can lower free T3. Intake of iodine, selenium and zinc matters for thyroid hormone production. Some medicines and large iodine supplements also affect the level.
Foods that help
Fish, seafood, dairy and eggs provide iodine; Brazil nuts provide selenium; pumpkin seeds and shellfish provide zinc.
Foods to limit
Very strict calorie restriction over long periods, and large amounts of iodine from supplements or seaweed.
Track it through the year
One value is a snapshot. Following it up shows whether what you changed is working.
Related markers
Educational information only — not a diagnosis. Every result is checked against safe limits, and anything critical is escalated for follow-up.
See all 84 markers →