The Free T4 (FT4) test measures the level of active thyroxine in the blood that is not bound to proteins. Thyroxine (T4) is one of the main hormones secreted by the thyroid gland, regulating various metabolic functions by entering body tissues. In the blood, T4 exists in two main forms: bound to proteins (bound T4) and unbound (free T4). The biologically active form that can be used by cells is free T4. This test is used to evaluate thyroid function and diagnose potential thyroid conditions such as hyperthyroidism or hypothyroidism. Free T4 levels outside the reference range can indicate various thyroid disorders.