Thyroxine (T4) is the main hormone created by the thyroid. Most of the thyroxine is inactive as it is bounded to proteins. Only small amount of thyroxine is unbound (free). To get an accurate picture of how the thyroid is functioning, the free thyroxine levels are measured. Thyroxine is important for the body as it is converted to triiodothyronine (T3), which helps to control the body’s metabolism. A low level of T4 means that thyroid is underactive, and a high level of T4 means that the thyroid overactive.