The T4 (thyroxine) is the main hormone produced by the thyroid gland. It helps the body to regulate energy and has also a role in controlling weight, body temperature, muscle strength, and nervous system.

The T4 Free (FT4) test measures serum thyroxine (T4) that is not bound to thyroid hormone-binding proteins and is thus readily available to target cells. This test is used with thyroid-stimulating hormone measurement (TSH) in the diagnosis of hyperthyroidism (usually displaying elevated FT4) and hypothyroidism (low FT4), including congenital and acquired hypothyroidism.

The FT4 test is used to assess thyroid function, monitor thyroid conditions, and assess treatment effectiveness. It has largely replaced the total T4 test, which measures both bound and free T4, because the FT4 test is less affected by levels of thyroid hormone-binding proteins.