Normally triiodothyronine (TS) circulates tightly bound to thyroxine-binding globulin and albumin. Only 0.3 % of the total T3 is unbound (free); the free fraction is the active form. In hyperthyroidism, both thyroxine (tetraiodothyronine; thyroxine: T4) and T3 levels (total and free) are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated.