Liothyronine sodium is a synthetic (man-made) version of one of the two hormones made by the thyroid gland, triiodothyronine. It is used for treating individuals who are hypothyroid (do not produce enough thyroid hormones). Thyroid hormones increase the metabolism (activity) of all cells in the body. In the fetus, newborn infant and child, thyroid hormones promote growth and development of tissues. In adults, thyroid hormones help to maintain the function of the brain, the use of food by the body and body temperature. The FDA approved liothyronine in May 1956.