Hormone Health Hub: Expert Insights on Testing, Balance & Better Living — T3 reverse T3 ratio

Reverse T3 (rT3) is one of the most misunderstood markers in thyroid testing. Often called the “hibernation hormone,” it is frequently claimed — without scientific basis — to block thyroid receptors and slow metabolism. In Part 2 of this series, Margaret Groves of ZRT Laboratory examines what the peer-reviewed literature actually says about rT3, when elevated levels are clinically meaningful, and how to interpret the T3/rT3 ratio in the context of a comprehensive thyroid assessment.

Thyroid symptoms can persist even when thyroxine (T4) levels appear normal — because thyroid function is profoundly affected by anything that disrupts the conversion of T4 to the active hormone T3 at the cellular level. In Part 1 of this two-part series, Margaret Groves of ZRT Laboratory explains the three deiodinase enzymes (D1, D2 and D3) that act as gatekeepers to intracellular thyroid hormone bioavailability, what affects circulating levels of T3 and reverse T3, and why selenium, iodine and inflammatory conditions all play a critical role.