The sodium salt form of liothyronine, a synthetic form of the levorotatory isomer of the naturally occurring thyroid hormone triiodothyronine (T3). Liothyronine sodium binds to nuclear thyroid receptors which then bind to thyroid hormone response elements of target genes.Liothyronine is used to treat hypothyroidism (a condition where the thyroid gland does not produce enough thyroid hormone).