Free Thyroxine ( Free T4) is the active form of the thyroxine hormone. Thyroxine (T4) is one of two major hormones made by the thyroid, a small gland located near the throat. The Thyroid helps regulate energy use, weight, body temperature, muscle strength, and the nervous system. This test helps diagnose, assess and monitor thyroid disorders, such as an overactive or underactive thyroid gland and aids diagnoses of pituitary gland disorders.