Free T3 (free triiodothyronine) is used to evaluate thyroid function and also in the assessment of abnormal binding protein disorders.  Measurement of Free T3 is valuable in confirming the diagnosis of hyperthyroidism, when an elevated free or total thyroxine level is found. Abnormal total and free triiodothyronine concentrations may appear in T3 toxicosis, in the presence of normal thyroxine levels. The levels of free T3 are unaffected by carrier protein variation.