Thyroxine (T4) is one of the two primary hormones produced by the thyroid gland. It plays a vital role in regulating metabolic processes in the body. T4 is mostly bound to carrier proteins in the blood, and only a small amount of it exists in its free or unbound form. In contrast to free T3, free T4 is relatively inactive, but it is still essential for normal body function. When the body requires thyroid hormone, free T4 is converted to the more active form, free T3.