Reverse T3 (rT3) is an inactive isomer of T3 produced when the body converts T4 along an alternative metabolic pathway. Rather than binding to thyroid hormone receptors and activating metabolism, reverse T3 is biologically inert — and may even block T3 receptors, further reducing thyroid hormone activity at the cellular level.

The body increases rT3 production as a protective mechanism during periods of physiological stress — including illness, surgery, starvation, and chronic psychological stress — to slow metabolism and conserve energy. However, persistently elevated rT3 in the context of chronic stress, high cortisol, or poor conversion can cause symptoms that mirror hypothyroidism (fatigue, brain fog, weight gain, cold intolerance) even when TSH and free T4 appear normal.