Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone thyroxine (T4) is activated by thyroid-stimulating hormone (TSH), which is released from the pituitary gland. This pathway is part of a closed-loop feedback process: Elevated concentrations of T3, and T4 in the blood plasma inhibit the production of TSH in the pituitary gland. Its effects on target tissues are roughly four times more potent than those of T4.

T3 stimulates the production of RNA Polymerase I and II and, therefore, increases the rate of protein synthesis. It also increases the rate of protein degradation, and, in excess, the rate of protein degradation exceeds the rate of protein synthesis. In such situations, the body may go into negative ion balance.