Useful For

Triiodothyronine is transported in serum primarily by thyroxine-binding globulin (TBG), approximately 99.5 % T3 is protein-bound. T3 contributes significantly to the maintenance of the euthyroid state and the total T3 concentration has a role in screening for thyroid disease in conjunction with other tests. T3 alone cannot diagnose hypothyroidism, but it may be more sensitive than thyroxine (T4) for hyperthyroidism. A fall in T3 concentrations of up to 50% is known to occur in a variety of clinical situations, including acute and chronic disease. T3 concentrations may be altered in conditions affecting the capacity of the thyroid hormone binding proteins, e.g., pregnancy, when a test for T3 or T4 uptake may be used with the total T3 result to calculate the free T3 index (FT3I) to estimate the level for metabolically active free T3.