List of products in the National Drug Code with proprietary name . Liothyronine is used to treat an underactive thyroid (hypothyroidism). It replaces or provides more thyroid hormone, which is normally made by the thyroid gland. Liothyronine is a man-made form of thyroid hormone. Low thyroid hormone levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. Having enough thyroid hormone helps you stay healthy. For children, having enough thyroid hormone helps them grow and learn normally. This medication is also used to treat other types of thyroid problems (such as certain types of goiters, thyroid cancer). It can also be used to test for certain types of thyroid disease. This medication should not be used to treat infertility unless it is caused by low thyroid hormone levels.