FREE T3 (FREE TRI-IODOTHYRONINE)

Overview

Normally triiodothyronine (T3) 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. FT3 levels may be required to evaluate clinically euthyroid patients who have an altered distribution of binding proteins (eg, pregnancy, dysalbuminemia). Elevated free triiodothyronine (FT3) values are associated with thyrotoxicosis or excess thyroid hormone replacement. Ref. Value: > or =1 year: 2.8-4.4 pg/mL.