reverse T3

Reverse T3 is the body’s way of making a hormone that does nothing on purpose. It looks almost identical to active T3 but has its iodine removed from the opposite position, so it cannot meaningfully switch on the thyroid hormone receptor.

Both T3 and reverse T3 are made from T4 by deiodinase enzymes; which one is produced depends on which iodine atom is removed. Generating reverse T3 is effectively a way to dispose of T4 without creating an active signal. During fasting, severe illness, or stress, the body shifts the balance toward reverse T3, which lowers metabolic activity and is thought to conserve energy when resources are scarce.

Despite being measurable in blood, reverse T3 has little place in routine thyroid diagnosis. Its level mostly reflects general illness rather than a specific thyroid disorder, and claims that a high reverse T3 proves hidden hypothyroidism needing treatment are not supported by good evidence. It is best understood as a marker of how the body is rationing thyroid hormone, not as a target to correct.