This test measures the amount of free triiodothyronine, or Free T3, in the blood. Free T3 is the not bound to a protein but free. Unlike T3 it is not affected by protein levels and protein binding ability. Measurement of Free T3 helps to diagnose hyperthyroidism, an excess production of Free T3, with symptoms such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes.