Triiodothyronine is abbreviated as T3, and its high level is a specific diagnostic indicator of hyperthyroidism, especially of T3 hyperthyroidism. However, since T3 exists in the blood in a form that is almost exclusively bound to thyroxine-binding globulin, any physiological or pathological factor that can affect globulin can lead to deviations in the value of T3. It is not scientific to judge hyperthyroidism solely on the basis of an increase in T3. Free triiodothyronine, or FT3, in the blood is an active part of thyroid hormone that is not affected by binding globulin and is therefore more sensitive and specific. Therefore, if the blood results of FT3 and thyrotropin are normal values and only a slight increase in T3 is present, it can be left untreated and regular monitoring of thyroid function is sufficient.