The free T3 (serum) test measures the level of triiodothyronine (T3) hormone in the blood that is not bound to proteins, making it biologically active. This test is used to evaluate thyroid function and diagnose conditions like hyperthyroidism or hypothyroidism. By measuring free T3, doctors can assess how well the thyroid is producing and regulating hormones, helping guide diagnosis and treatment for thyroid-related disorders.