As the most common brand name for a synthetic thyroid hormone, Cytomel is also, and more specifically, a synthetic version of T3 (triiodothyronine ). Rather than a direct production of T3 by your thyroid gland directly, T3 is actually transformed from the T4 thyroid hormone (thyroxine).
Between two primary thyroid hormones, T3 functions as the more metabolically active. It is also regulates oxidative metabolism of energy producing substrates through your cell’s mitochondria. These mitochondria produce ATP that the body uses as energy. In this instance, since T3 supplementation causes an increase in ATP, you will increase overall metabolic activity by using Cytomel. This is done in part by increasing the volume of blood pumped by your heart, which will put demands on your ATP usage. In addition to that, this increased heart rate and increased force of contraction of the cardiac muscle accounts for 30-40% of ATP usage in abnormally elevated thyroid levels.
Furthermore, T3 also may cause the body to increase its production of uncoupling proteins, which are a transporter family present in the mitochondrial inner membrane. Uncoupling proteins serve to separate respiration from ATP synthesis by expending the transmembrane proton gradient as heat. This greatly inhibits the body’s ability to produce useful energy. This is because heat is generated instead of ATP being produced from energy substrates, without providing the body with useful energy. As you may suspect, this means you are burning calories without actually producing constructive energy. More specifically, calories are being burned even when you are just sitting around. When you actually need to produce energy from foodstuffs, you have already burned off a fair portion which was released as unnecessary heat. This uncoupling most likely takes place in skeletal muscle and is one mechanism through which T3 (Cytomel) can cause increased basal metabolic rate