T3 (Triiodothyronine, Total T3) is one of the two main hormones produced by the thyroid gland and is activated by the thyroid-stimulating hormone (TSH) released from the anterior pituitary gland. Increased T3 concentration inhibits the production of TSH in the anterior pituitary, whereas decreased T3 concentration increases the production of TSH. This negative feedback control system stabilizes the amount of thyroid hormone in the blood. T3 is a hormone with higher metabolic activity than T4 and is known to have a stronger effect on target cells. Of the thyroid hormones produced, only about 20% are T3, 80% are produced as T4, but about 85% of the circulating T3 is produced by removing the iodine atom bound to the carbon atom number 5 of the outer ring of T4 from the liver or pituitary anterior lobe. T3 not only increases the basal metabolic rate, increasing the body's oxygen and energy consumption, but also plays a very important role in maintaining thyroid function.