The Triiodothyronine (T3, Total), serum helps to evaluate the function of the thyroid gland, diagnose diseases of the thyroid, and monitor the effectiveness of ongoing thyroid treatment. The Triiodothyronine (T3, Total), Serum test is done: • In case of abnormal levels of TSH especially when Free T4 levels are normal • In case of signs and symptoms of hyperthyroidism