Definition
Triiodothyronine (T3) is an active thyroid hormone. It's produced in smaller amounts by the thyroid gland, but mostly converted from thyroxine (T4) in other tissues. It's much more potent than T4. T3 binds to receptors in cells and affects metabolism. It is the main driver of thyroid hormone action. Unlike insulin (regulating blood sugar), T3 controls metabolic rate. It helps your body use energy efficiently.