Free T3 measures the free, unbound levels of triiodothyronine in your bloodstream. Free T3 is considered more accurate than Total T3. Free T3 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.
Most of the thyroid hormones circulating in the blood are bound to carrier proteins. This test measures the level of the active thyroid hormone T3 which is free (not bound to protein) and therefore available to your cells to regulate metabolism. Raised levels of FT3 indicate an overactive thyroid whereas low levels indicate that your thyroid is not producing enough thyroid hormones.
Other factors may influence your FT3 levels, such as your ability to convert T4 to T3 as well as the number of carrier proteins in your blood stream: raised levels of proteins may mean that you have insufficient FT3 available for your cells.
This test is for people who have symptoms of an abnormal thyroid function or who wish to check an already diagnosed condition. This test is often used as a follow-up to an abnormal thyroid test to gain further insights into the diagnosis.