Reverse T3 (rT3) is a form of thyroid hormone produced by the body, but unlike T3, it is considered inactive and doesn't play a significant role in regulating metabolism. At Tendwell, we consider the measurement of reverse T3 levels to be an important aspect of comprehensive thyroid testing, which is, in turn, an important component in many holistic health plans. In some cases, an elevated rT3 level can be an indication of a problem with the conversion of the inactive form of thyroid hormone (T4) to the active form (T3). This is important because T3 is the hormone that has the most metabolic activity and is responsible for regulating many body functions such as heart rate, body temperature, and energy levels.