Triiodothyronine (also known as T3) is a thyroid hormone used for the treatment of hypothyroidism and greatly affects a wide range of physiological processes in the body, including growth and development, metabolism, body temperature, and heart rate. T3 is involved in many cellular functions, including the metabolism of carbohydrates, fats, and proteins. Losing fat is extremely difficult when suffering from hypothyroidism; moreover, it is very common to gain body fat with this condition. Also, those suffering from thyroid hormone deficiency often experience a loss of energy. By supplementing with T3, the problem is solved by providing the body with the thyroid hormone it needs. T3 increases the base metabolic rate and, thus, increases the body’s oxygen and energy consumption. It increases the rate of protein degradation so that the body may go into negative nitrogen balance, and it also potentiates the effects of the β-adrenoreceptors on the metabolism of glucose. Therefore, it increases the rate of glycogen breakdown and glucose synthesis in gluconeogenesis. T3 stimulates the breakdown of cholesterol and increases the number of LDL receptors, thereby increasing the rate of lipolysis. T3 increases heart rate and the force of contraction, thus increasing cardiac output by increasing β-adrenoceptor levels in the myocardium. This results in increased systolic blood pressure and decreased diastolic blood pressure.

Many athletes who use performance-enhancing drugs enjoy utilizing triiodothyronine to enhance fat loss. The use of T3 along with a calorie-restricted diet will promote significant and accelerated fat loss. While this approach may be effective for losing weight, it generally cannot be recommended without the use of anabolic steroids. Without the help of an anabolic agent, the relatively higher T3 doses needed to promote enhanced fat loss and the accompanying depletion of ATP (adenosine triphosphate), can lead to lean tissue loss. In other words, the body will take whatever it needs from wherever it can get it to meet its energy demands. For this reason, an anabolic protectant is normally advised when supplementing with T3.