Uses: Liothyronine is used to treat underactive thyroid (hypothyroidism). It replaces a hormone that is normally produced by the thyroid gland. Low thyroid levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. It is important to have adequate levels of thyroid hormone in your bloodstream to maintain normal mental and physical activity. This medication is also used to lower thyroid function in certain diseases such as enlarged thyroid gland (goiter) and Hashimoto's thyroiditis. It is also used to test thyroid activity. Liothyronine is a man-made hormone that replaces the body's natural thyroid hormone (T3).