A FT3 blood test determines whether the thyroid is performing properly, and is used mainly to help diagnose hyperthyroidism, since FT3 can become abnormally high earlier than FT4 and return to normal later than FT4. FT3 is not usually helpful if your doctor thinks you have hypothyroidism.
What is the difference between T3 and FT3?
T3 has 2 forms: bound and free. Bound T3 is attached to a protein and free T3 is not attached to anything. The free T-3 test measures only the amount of free T3. The total T3 test measures both free and bound T3 in your blood.