rT3 - Reverse T3
Reverse T3, or rT3, is often considered as a metabollically inactive form of T3. Technically it is the mirror image of T3 and therefore may have a role in blocking some of the action of T3, in effect this has a 'negative' metabolic action and acts as if applying metabloic brakes.
Reverse T3 may be raised at times of acute or chronic stress or illness and can reduce the action of active T3 in the body.
There is controversy about the role of reverse T3 in thyroid illness. One view is that reverse T3 is effectively 'antithyroid' compared to 'standard' T3 which is the active thyroid hormone that stimulates the body's energy system (metabolism). Reverse T3 is a mirror image and when it goes into cells, instead of controlling metabolic processes it blocks the effect of T3. In this model, reverse T3 is effectively a hibernation hormone - in times of stress and chronic illness it lowers energy release from the cells - so normal thyroid levels with high reverse T3 may still result in a hypothyroidism-like pattern.