Test Details: This test measures the amount of fT3 in the blood. Usually prescribed if there is an abnormal TSH or FT4 results. Helps to diagnose hyperthyroidism and monitor therapy. T3 is one of the two major hormones produced in the body. T4 is used as a reservoir. Around 99.7% of T3 in blood is attached to a specific protein and the rest is free. High free T3 may indicate an overactive thyroid gland. Low free T3 may indicate an underactive thyroid gland.