Reverse T3
An inactive form of thyroid hormone that blocks cell receptors, slowing down metabolism during times of stress, illness, or caloric restriction.
Deep Dive
Think of Reverse T3 (rT3) as the 'anti-metabolism' hormone. While Free T3 hits the gas pedal on your cellular energy production, Reverse T3 hits the brakes. It is produced when the body diverts T4 conversion away from active T3 and towards this inactive mirror-image molecule.
Elevated rT3 is the biochemical signature of the Hibernation Response. It explains why you can have 'normal' TSH levels on a standard blood test but still feel cold, tired, and unable to lose weight. Most GPs do not test for Reverse T3, which is why millions of people with thyroid dysfunction are told their labs are 'fine.'
Common triggers for elevated Reverse T3 include chronic caloric restriction, prolonged fasting, sustained psychological stress, over-exercising, inflammation, and certain medications. The solution is not more thyroid medication — it is addressing the underlying stressor that is causing the body to apply the metabolic brake in the first place.