Thyroxine (T4) is a major hormone produced by the thyroid, a small gland in the neck. The Thyroid helps control energy use, weight, body temperature, muscle strength, and the nervous system. This test is used to detect and monitor thyroid disorders, such as an overactive or underactive thyroid gland. It also helps diagnose pituitary gland disorders. Thyroxine (T4) makes up most of the thyroid hormone, while triiodothyronine (T3) makes up less than 10%.