Free T4
Free T4 is the main hormone released by the thyroid gland; the body converts it into the active hormone T3.
Free T4 (thyroxine) is the main hormone the thyroid gland releases. The body converts it into T3, the active form, which influences metabolism, energy use and body temperature. “Free” means the part not bound to proteins in the blood. Retesting shows how it changes over time.
Intake of iodine, selenium and zinc matters for thyroid hormone production. A recent illness, stress, pregnancy and some medicines can shift free T4, as can large iodine supplements.
Foods that help
Fish, seafood, dairy and eggs provide iodine; Brazil nuts provide selenium; shellfish and pumpkin seeds provide zinc.
Foods to limit
Large amounts of iodine from supplements or seaweed, and very strict calorie restriction over long periods.
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 →