Cytomel is not a steroid, but more a of a cutting aid. It’s 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 mcg for mcg basis.