Reverse T3

Reverse T3 (rT3) is a metabolite of thyroxine (T4) that cannot be directly converted from T3, despite its name suggesting otherwise 1. It is primarily produced in the liver through the action of the deiodinase enzyme, which is activated by cortisol 3. High levels of rT3 can be a sign of a catabolic state, resulting from a decreased peripheral metabolism of thyroid hormone due to aging or disease 1.

The production of rT3 can also be influenced by other factors, such as malnutrition or the presence of certain enzymes, and it is not solely a result of taking T4 medication 2. In fact, taking a combination of T3 and T4 can lower rT3 levels 2. Additionally, rT3 has a lower affinity for thyroid receptors compared to T3, making it less effective in binding to these receptors 6.

The brain has a higher concentration of T3 compared to T4, and it can achieve a higher concentration of T3 than T4 4. Reverse T3, on the other hand, has a lower ability to enter the brain, but increased T4 can cause it to be produced in the brain 4.

In some cases, high rT3 levels can be a sign of a problem with thyroid function, even if other thyroid tests appear normal 6. A supplement of T3, such as Armour Thyroid or Thyrolar, can help to quickly alleviate symptoms of high rT3 levels 8.