Free Thyroxine (FT4)
When the thyroid stimulating hormone (TSH) test result is above or below the normal range, a free thyroxine (FT4) is performed to help make an accurate diagnosis. Thyroxine (T4) can either be bound to proteins or be unbound. The unbound portion is called FT4 and is the biologically active form of thyroid hormone that controls the rate of metabolism. Elevated levels of FT4 are seen in hyperthyroidism or an overactive thyroid gland. Conversely, decreased levels of FT4 are associated with hypothyroidism or underactive thyroid gland.