Thyroxine (T4) is one of two major hormones produced by the thyroid gland; the other one being triiodothyronine (T3). T4 makes up nearly 90% of thyroid hormones, while T3 makes up less than 10%. Inside the thyroid gland, T4 is bound to a protein called thyroglobulin. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound), or protein-bound (primarily bound to thyroxine-binding globulin). The concentration of free T4 is only about 0.1% of that of total T4. Since free T4 is the active form of thyroxine, it is thought to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4. Thyroxine’s principal function is to stimulate the consumption of oxygen and thus the metabolism of all cells and tissues in the body. This test is usually ordered when a patient has symptoms of hyper- or hypothyroidism (such as weight gain/loss, dry skin, irregular menstruation, increased heart rate, anxiety…etc.), or to monitor the effectiveness of treatment. In general, HIGH free or total T4 results may indicate an overactive thyroid gland (hyperthyroidism), or thyroxine overload. LOW free or total T4 results may indicate an underactive thyroid gland (hypothyroidism), and certain serious conditions not related to the thyroid gland.