Liothyronine is a thyroid hormone replacement therapy used to treat hypothyroidism, to suppress TSH, and to help in the diagnosis of hyperthyroidism. Liothyronine is a thyroidal hormone T3 which is normally produced by the thyroid gland in a ratio 4:1 when compared with T4: T3. Liothyronine is the active form of thyroxine which is composed in a basic chemical structure by a tyrosine with bound iodine.