T3 is the metabolically active form of thyroid hormone, produced by deiodination of T4. Mostly in liver, kidney, and target tissues.
Mechanism: 4–10x more potent than T4 at the thyroid receptor. Free T3 is the unbound fraction available to cells; it's the hormone that actually drives metabolic rate, body temperature, and cellular energy production.
Lab range: 2.3–4.2 pg/mL standard; many practitioners target the upper third.
Caveats: Low fT3 with normal TSH and fT4 = peripheral conversion problem (often related to selenium, iron, cortisol, or chronic illness). T3-only or T3/T4 combo therapy (NDT, liothyronine) addresses this.