Liothyronine sodium is a synthetically manufactured prescription thyroid hormone. It specially consists of the L-isomer of the natural thyroid hormone triiodothyronine (T3). Thyroid hormones stimulate basal metabolic rate, and are involved with many cellular functions including protein, fat, and carbohydrate metabolism. Liothyronine sodium is used medically to treat hypothyroidism, a condition where the thyroid gland does not produce sufficient levels of thyroid hormone. Hypothyroidism is usually diagnosed with a serum hormone profile (T3, T4, & TSH), and may manifest itself with symptoms including loss of energy, lethargy, weight gain, hair loss, and changes in skin texture. T3 is the most active thyroid hormone in the body, and consequently liothyronine sodium is considered to be a more potent thyroid medication than levothyroxine sodium (T4).

Bodybuilders and athletes are attracted to liothyronine sodium for its ability to increase metabolism and support the breakdown of body fat. Most often utilized during contest preparation or periods of “cutting”, the drug is usually said to significantly aid in the loss of fat, often on higher levels of caloric intake than would normally be permissive of such fat loss. To this end, the drug is also commonly used in conjunction with other fat loss agents such as human growth hormone or beta agonists. Some users also ascribe an ability of thyroid hormones like liothyronine sodium to increase the anabolic effect of steroids. While in theory these drugs may support the greater utilization of protein and carbohydrates for muscle growth, they are not widely proven or accepted for this purpose.