Cytomel is a synthetic form of the thyroid hormone tri-iodio-thyronine or T3, made up of a metabolite of the amino acid tyrosine and 3 iodine ions. In the body it in turn is made from another hormone, T4, which is secreted by the thyroid under influence of the pituitary hormone TSH (Thyroid stimulating hormone). If a shortage of either TSH or T4 is noted, usually doctors may opt for a replacement therapy. These days the most common prescription is synthetic T4 (synthroid), but in more severe cases of permanent thyroid dysfunction, the choice is given to Cytomel. Simply because T4 is mostly active through its conversion to T3 and T3 is 4-5 times stronger than T4 on a µg for µg basis.

In bodybuilding Cytomel is mostly used as fat-loss drug. Thyroid hormones are often referred to as the metabolic regulators of the body. High levels of T3 speed up the metabolism of an individual, allowing him to burn more calories and use calories more sufficiently.

Side effects may include headache, skin rash or hives, chest pain, confusion, fast or irregular heartbeat, mood swings, muscle weakness, psychosis , restlessness (extreme), yellow eyes or skin, shortness of breath.