This test is for diagnosis of hypothyroidism and hyperthyroidism.
FT4 (free thyroxine) is the active form of thyroxine, released from the thyroid gland in the neck. Its role is to control metabolism - release of energy from all cells of the body. A lack of thyroxine leads to, for example, tiredness and weight gain, whereas an excess will lead to weight loss, rapid heart rate and anxiety. It is generally believed that this level (FT4) is the most reliable indicator of thyroid status - ie whether the gland is over-active (hyperthyroidism) or underactive (hypothyroidism).