T3 is a synthetically manufactured thyroid hormone. Its effect is similar to that of natural L-thyroxin (L-T4) in the thyroid gland. T3 is one of two hormones which is produced in the thyroid. The other one is L-triiodthyronine. Bodybuilders use T3 to accelerate the metabolizing of carbohydrates, proteins, and fat. The body burns more calories than usual so that a lower fat content can be achieved or the athlete burns fat although he takes in more calories. When used properly there are few side effects to T3.