Cytomel is a synthetic thyroidal hormone designed to increase the metabolic rate in the cells. On Cytomel, the cells mitochondria will take in more nutrients and burn them quicker. Bodybuilders use this compound to burn off fat and give them a more chiseled look. Although reports of natural thyroidal hormones shutting down from taking synthetics are often exaggerated, it is recommended that users take some time off from them to allow the body to go back to natural production for at least three to four months a year.