Liothyronine is used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormone. It is also used to help decrease the size of enlarged thyroid glands (known as goiter).
The thyroid produces a hormone called triiodothyronine, known as T3. It also produces a hormone called thyroxine, known as T4. Together, these hormones regulate your body's temperature, metabolism, and heart rate.