Free T3 (triiodothyronine) is the most metabolically active thyroid hormone. While T4 is produced in larger quantities by the thyroid gland, T3 is the hormone that actually drives metabolic rate at the cellular level. Most T3 comes from peripheral conversion of T4 to T3 by deiodinase enzymes.
Low free T3 with normal TSH can indicate poor T4-to-T3 conversion, which may respond to lifestyle interventions or supplementation rather than thyroid medication.