The Free T3 test is a blood test that measures the active, unbound form of the triiodothyronine (T3) hormone. This test is used to evaluate thyroid function, diagnose hyperthyroidism, and in cases where a thyroid disorder is suspected. It is also useful in differentiating hyperthyroidism from non-thyroidal illnesses. Free T3 levels, determined by a blood test, may indicate an abnormal thyroid condition when outside the reference range.