Liothyronine is in a class of medications called thyroid agents. It is used to treat hypothyroidism. It supplements or substitutes thyroid hormone, which is typically produced by the thyroid gland. Liothyronine is a synthetic thyroid hormone. Having an adequate supply of thyroid hormone aids in your overall wellness. Liothyronine comes as a tablet to take by mouth. It usually is taken once daily. Cytomel may cause serious side effects including headache, irritability, trouble sleeping, nervousness, increased sweating, heat intolerance, diarrhea, and menstrual changes.