Reverse T3 is an inactive form of T3 that the body produces from T4 instead of active T3 under certain conditions. It binds to thyroid receptors but doesn't activate them, effectively blocking active T3 from working.
Elevated reverse T3 is a sign that the body is intentionally dampening thyroid activity, a common response to chronic stress, severe illness, calorie restriction, or inflammation. A high rT3 with normal TSH can explain hypothyroid symptoms despite "normal" thyroid testing. The T3-to-Reverse T3 ratio is a more sensitive marker of tissue-level thyroid function than TSH alone. Reverse T3 testing is less common in standard panels but particularly valuable when symptoms don't match basic thyroid results.
Optimal Reverse T3: 10-20 ng/dL.
Typically re-tested every 10 weeks.